You may walk along Bollwerk, a busy, noisy street, pass underneath the train tracks, and continue till you hit the INF.
Alternatively, you may take the elevator (next to track 12) up from the train station to the Grosse Schanze, a park in front of the main University building. On a clear day, there is a beautiful view of the alps and of parts of Bern. From there, walk past the Science building ("Exakte Wissenschaften") till you hit Sidlerstrasse and find the footpath down the hill to INF. You will easily recognize the yellow brick building.
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If you are staying overnight at the Hotel Kreuz or the Hotel Metropole, they are just a few minutes from the train station. Get out of the train station by the Neuengasse exit, walk towards Waisenhausplatz, and then you will find the Hotel Kreuz at Zeughausgasse 41 and Hotel Metropole at Zeughausgasse 26.
To get to the INF from the Hotel Kreuz, just backtrack and walk along Bollwerk to the INF.