I admit it. I've not been in the museum but the park is itself like a very grand Open Air Museum with [literally] hundreds of Vigeland sculptures, most in stone some some excellent bronzes over the bridge as you enter.
However you get to oslo, you're almost certain to find yourself near the station. There's a transport info. office just outside who will give you a transport map. T-bahn + bus will be best for reaching the park.
Top and left clicks respectively will give you info on the park and museum from the website - in English [or Norwegian, of course].