Train station - Möllan is about 3 km, and Möllan - Stadium about 2 km. Either you can walk the pedestrian zone from Stortorget - Gustav Adolfs Torg - Triangeln - Möllan, or just catch a bus (line 5, 7, 34 and 35 amongst others) from the station for £2. I think the easiest way to get to the stadium from Möllan is by foot - just follow the MFF fans and you will get there.