Tobias,
I didn't know that you were wokring with Creo Direct. Unfortuantly at this time, the student license of Creo 2.0 Direct does not include Creo Direct Assemblies. That said, you should have no problem doing assemblies in Creo 2.0 Parametric.
Sorry for the inconvenience, please let me know if there's anything I can do to assist at all!