The closest bus stop is called Arendal Skans.
If you’re coming from the city centre, the easiest way to get here is to first go to Eketrägatan (tram 5, 6 or 10). Then change to bus 32 towards Volvo Torslanda (via Arendal).*
Get off at Arendal Skans and follow the signs. The museum is located about 100 metres from the bus stop (close to the water).
* N.B. On weekends, you need to call ahead and book bus 32 for it to depart. Call Västtrafik (the public transport company) at +46-31-788 07 89 at least one hour before the departure time.