SAS Scandinavian lounge in domestic terminal 4 (no ticket needed to enter this terminal), itīs located just after security on the left side. Itīs even possible to enter this lounge without a same day ticket since this lounge can be entered by just draging the card thru a automatic check point.
SAS business lounge is located to the left right after security check at pier B in terminal 5 (ticket needed to enter).

For a complete list of ARN lounges see this: http://www.lfv.se/site/airports/arla.../on/lounge.asp