I was in Stockholm last december; from Arlanda (Terminal %) to downtown you can find a fast train (expensive) and a cheaper bus (Flygbussarna).
Bus leave airport every 10 minute, approx., and in 40 minutes, without any big traffic problem, you can arrive to the World Trade terminal, near Central Station.
You can buy the ticket directely on the bus, but paying with credit card only.
438 SEK (48 EUR approx.) return ticket. about 6 years ago