u don't need to be an adea member to use AADSAS. u shouldn't register for AADSAS until this cycle is over. i think you can register for AADSAS as early as May of next year.
how to take the DAT's? pick up a sample DAT booklet from the ADA. it should be available in ur career center. take a look at those practices, it'll give you a feel for the test (well, not really, since the test is on computer, not paper). from there, i'd go look for study dat study material. there are all sorts: kaplan, dat achiever, topscore, etc.
good luck.