Hi Mikem409. You can wear whatever you like - not everyone dresses in traditional Bavarian clothing for the festival; there are plenty of people there in jeans and t-shirts too. Don't worry!
If you want to get into the spirit of things and dress like the locals, then it'd be best to wait until you arrive and check out one of the discount Trachten shops around Munich. There are a few recommendations already on this forum, so do a search for Tracht or
Lederhosen.