Address of the Main Hotel/Venue
Scandic Infra City
Obviously the best and classiest way is to take the FuzzBus from Arlanda to the hotel. There will be a meeting place for all early arrivals to start meeting up and spend the time until boarding for NFC at Grand Café in level 2 of the Terminal 5 arrival area, with a map available here.
If you cannot make it to the FuzzBus or were too late to get the popular tickets.
The easy way is to get a cab. A cab should cost approximately 350 SEK for up to four people and 560 SEK for up to 7 people; ask for the price for Scandic Infra City - before you start driving! - and you should get a quote. There's no need to go for the first cab, you can ask around, but they should more or less give you the same price. Price quotes and further ordering information may be found at Taxi Stockholm, Taxijakt.se, or Taxi020's web pages.
If you feel a little more frugal or wish to try out the public transport of the capital of Scandinavia you can get a ticket for the bus next to the train stop in the middle of Sky City. Then you get out from the airport and get on bus 583 to Märsta. It goes to the end of the line which is a train station, get on the next train leaving and go for 2 stations to Upplands Väsby. There is a slightly faster alternative by going directly on the train towards Stockholm C (additional airport fee), but remember to get off in Upplands Väsby, which is the next stop. The bus option is cheaper and only slightly more annoying. Both alternatives get you to Upplands Väsby from where you can take a bus to Scandic Infra City (see below).
Get a ticket for SL in the stores called Pressbyrån, any of the SL ticket machines or ask around or at the train station. Then board train 36 (or 40) towards Upplands Väsby. It is near the end of the line so it takes a while. If you plan to spend time in other parts of Stockholm, now or in the future, check below at Ticket advice. There is a slightly faster way to go via Rotebro but unless you time the buses you will on average not arrive earlier.
From Upplands Väsby you board bus 536 towards Bollstanäs or 566 towards Infra City Väst.
If you intend to go anywhere else, like downtown or live at a different hotel, you might consider a 24 hour, 72 hour or 7-day ticket instead. They let you travel all over Stockholm, even the SL boats in the city.
Travelling in Stockholm is done using SL. On their website you can find a travel planner. The bus stop outside the main hotel is called: Infra city (Upplands Väsby).