On the Campus map you find the A wing in the bottom-left corner of the central building. The tram station of line 4 is at the bottom of the map („Haltestelle Universität“). From there, a bridge takes you to the main entrance („Haupteingang“).
Tram line 4 stops at various points near the city center (among them the main station) as you can see on the map of public transportation and the tram line schematics. For further information on public transport in Bielefeld, please refer to the website of its provider MoBiel.
The restaurant Glück und Seligkeit for the conference dinner is at the Artur-Ladebeck-Straße 57. The next tram stop is Bethel on line 1.
Please consult the corresponding website of the university for travel information.
Note that while on that page Paderborn/Lippstadt and Hannover are mentioned as the closest airports, you might also consider flying to Düsseldorf (~ 2 hrs by train from Bielefeld) or Frankfurt am Main (~ 3-4 hrs by train from Bielefeld), which offer much better overseas connections.