As far as I know, you can't. Calendar sync wasn't included on the mobile apps. Those of us who don't have access to MLO 5 throughout the day need it in case we need to make changes.
As a workaround for adding tasks though, what I do is I add events in GCal and treat them like tasks if I need something that must be done at that time. At the end of the day, if the 'task' is still didn't get done, I will edit the event, switching it from my personal calendar in GCal to the MLO calendar in GCal. Then I perform a calendar sync in MLO 5. That way, that tasks and its notes come over to MLO 5.
The downside is, of course, that I can't add tasks that already exist in MLO Android to GCal when I'm out and about which is the problem that calendar sync would solve which is why we need it.
Joel