If you're talking about the email about medical services volunteers needed for test events then I was under the impression that it implied you could take part in any test event regardless of whether or not you'll actually be at that venue during the Olympics and that the venue training provided will be specifically for that event meaning you'll probably still have to attend venue training for your Games Maker role even if it is in the same place that you end up doing a test event.
You can do any of the test events mentioned in the email but if you plan to do a test event then you have to do the venue training that is set with that event. So if you want to do the test events at the Olympic Stadium, you'll have to go to the venue training planned a couple of days beforehand. It's part of the London prepares series and doesn't have much to do with the whole Games Maker scheme unless you have a specialist sports role in which case it is necessary to do a test event when asked.
This is completely separate from being a Games Maker and the venue training provided won't be part of your Games Maker training.
And yes, I'm guessing that in order to do a test event in the Olympic Stadium, you'll have to go to the Olympic Stadium venue training provided for the event. Olympic Park venue training wouldn't be suffice since it's the space in between all the venues and therefore won't include much detail about the Olympic Stadium.