Getting from the Airport to the Hotel
The "hotel location" page has some details under "How to get to the hotel"
If you want the easy and quick but expensive option, on exiting the airport you should see the taxi stand for Főtaxi. Tell them the hotel, and they'll give you a fixed price for the taxi, which will probably be around HUF 9,000 / €30. Journey time is around 30 minutes. It is not advised to take any taxi offered to you by touts in the airport arrivals area or just outside, go with the official operator Főtaxi with their pre-agreed prices.
By public transport, first thing to do is get a ticket. Just outside of the Customs area is a BKK Transit Desk, who sell public transport tickets. Depending on the amount of walking you want to do, you will need one of 3*single tickets (3*HUF350), a combined bus+metro (transfer / átszállójegy) ticket for HUF 530, or some sort of longer pass if you want to travel around a lot during the conference. (3 day / 72 hour travelcard is HUF 4,150, 7 day travelcard is HUF 4,950, 5 single days in a 30 day period card is HUF 4,550)
Once you have bought your ticket, head outside and catch the 200E bus (it's the only bus from the airport). Take the bus to the very last stop, Kőbánya-Kispest, should be about 25 minutes. Change here for Metro 3 / blue line. Your route + tickets would be:
Minimise walking - 200E bus, Metro 3 / blue line for 7 stops to Ferenc korut / Corvin-negyed station. Exit here, and take Tram 4 or 6 to Kiraly street stop (Király utca), which is by the hotel. Needs 3 single tickets or a daily/longer passes. Transfer / átszállójegy tickets are not valid for bus+metro+tram combinations
- Cheaper route - 200E bus, Metro 3 / blue line for 10 stops to Deák Ferenc tér, then Metro 1 / yellow for 3 stops to Oktogon. Walk south-east along Teréz krt for 200m to the hotel. Can be done on a HUF 530 transfer / átszállójegy ticket, or daily/longer passes
Overall journey should be between about 50 minutes and 1 hour 10 minutes, depending on route taken and connection timings.
Getting from the Railway Station to the Hotel
Most international trains arrive into Budapest's historic Keleti (east) station. From there to the hotel is either a 10 minute taxi ride away, or about 15 minutes by public transport. Take either the Metro 2 / red line one stop to Blaha Lujza square, or Metro 4 / green line two stops to Rákóczi tér. Either way, it's then Tram 4 or 6 until Kiraly street stop (Király utca). You'll need a Transfer ticket (átszállójegy) for the journey, costs HUF 530. Alternately, it's about 1.5km if you want to walk!
Budapest Visitor Information
Where to eat near the hotel
Where to drink
Where to play
Where to stay (if the conference hotel isn't your thing)
Get out - where to visit nearby
Thinking of heading to Budapest a few days early? Or maybe staying a few days more? Beyond some great things to do in the city, it could be worth planning a day or two's exploring nearby.
Unsorted information, from a fellow Apache member
The Budapest Zoo  is a great visit (if you enjoy zoos), and they have an additional themepark (where there used to be a very old crufty themepark named 'Vidam park'). If you get a chance (and if it is still around) a ride in the Hullámvasút wooden rollercoaster is really something. Last time I was in it, it had a driver manually controlling the brakes when the train hit 60km/h.
There are several escape rooms for indoor entertainment.
You can get a guided bike tour  through Budapest (great way to do the sight seeing) from the square in front of the Szent Istvan Basilika (highly recommended, especially if you go across the bridge to Buda and up the hill).
The Buda part (the hill's) are also nice to visit and climb, they have a cavern system (the labyrinth ) you can walk through with some entertainment for children. The view from the Fisherman's Bastillion is awesome.
You should pay a visit to the thermal springs (I'd recommend the Szecheny Fürdö baths, in the same park as the Zoo and vidam park)
A couple of years ago there was a technological exhibit where you (i.e. your 10 yr old) can climb, and do sorts of science experiments but I don't recall the name and don't know if it is still around.
Just go to an ATM and get it from there. Those accept maestro, mastercard, visa etc. What the websites told me is that the yellow cabs are required to have an electronic payment terminal. Chip and pin should be supported (maestro), but I think my Budapest friend usually pays by cash or credit card. Don't keep too much cash at hand as it is next to worthless when you leave the country, but it is great for scrap booking :-D
The streets in Budapest are (in the center) pretty safe, just use your normal precautions. My friend living in Budapest treats it like he lives in Mexico City or Juarez, but I think the crime level is on par with any city with about 2M inhabitants. I don't feel more unsafe in Budapest than in Amsterdam.
Traffic is a bit hectic as people drive fast. The metro system is a great way to go about the city, and you put the trams to good use as well.
For public transport, note that Metro Line 4 opened in March 2014, and with that quite a few bus and tram routes were re-numbered or re-organised. Don't trust any map that only shows 3 metro lines, make sure you're using one that's up-to-date!
Other helpful information sources
thanks to airberlin, here is a city guide, with food places etc: city guide
Other Open Source Events
Help needed! What other Open Source events are on in Budapest at the same time?