Well, I got it working. Don't know for sure what the essential part was as I did two things:
First, I downloaded, installed and registered m:IQ, synchronizing everything.
Second, I changed my mobile settings on Google Calendar. I hadn't verified my mobile phone yet.
Now, when I add an entry on the calendar on my BB it is added to the calendar provided by Google and vice versa.
edit: And by the way, this only applies to new entries. The existing entries in the calendar on my BB were not added to Google Calendar so I'll have to add them again for complete synchronization.