Sync iCal on iPod Touch
Depends what calendar you're adding the events to. In the Calendar app, in the top left, there is a Calendars button which lists all the calendars on the phone. You should hopefully see your MobileMe calendars in there, labelled as such in their own section. The theory goes that you should see any events on MobileMe that you've added to the calendars listed under the MobileMe section of the screen.

I would guess you're adding the events on the iPod touch to the wrong calendar, but it could be there's a separate issue. You'll need to check what Calendar is selected on the Event screen when you're adding or editing the event.