If you come to Bergen by plane, take the Airport Bus to the Bus Terminal (not its last stop!).
On the rightmost half of this map you'll be able to locate the train station (Jernbanest.) and the Bus Terminal (Bystasjonen), marked with J and I, respectively, as well as the location of the Department of Mathematics, which is located in Johs Brunsgt 12 (the street but not the building is mentioned on the map, but the street is very short. If you arrive via Nygaardsgaten, you'll see a place where there used to be a gas station (no longer there, unfortunately); then the math dept is in the same building but with the entrance on the other side.) You may also consult this campus map, where you can click on the number 38 to get this (look at the rightmost half of the map).
To get from the Railway Station or Bus Terminal to the Park Pension Hotel, Harald Hårfagres gate 35, follow Strømgaten to its very end, 5-10 minutes' walk.
Other maps, blowups of the first one: