How do you guys create Mods? Do you use MS Access? Just curious

Depends what kind of mod your talking about (i.e. data changes, graphics, photos, etc.). To edit the data in default.dat you need a database program capable of reading/writing MS Access databases. Look at this recent post on two FREE programs that can do this if you don't have MS Access. I personally use MS Access since I own it, but would have no issue using the free stuff either. If your looking to do graphics and such, that is a different thing.