After installing 3G Unrestrictor all you have to do is start the 3G Unrestrictor config app, tap the "Edit" button and then the + button. You will be presented with a list of all of your installed applications. Choose one by tapping it. You can also type in the name in the search box to search directly for an app.

After that 3G Unrestrictor will automatically trick the app into thinking it is on WiFi.