I don't know of an "expert" mode in Pronterface. There is, however, an expert mode in Slic3r that is packaged with Pronterface. If you do not know how to open Slic3r. In the Pronterface panel, choose "settings" tab and click "slicing settings". In the Slic3r console, choose "file", in the drop down menu click "Preferences...". Here you'll be able to change the "mode" from Simple to Expert.
