Ok, I do this sort of thing a lot, but I haven't done it with Access as my source of data. In your data connection to Access, are you connecting to the MDB itself or to individual tables? If it's to individual tables, then do you have 2 separate connections - one for each table that has data? If it only connects straight to the MDB, then are you able to specify tables when retrieving data?
Now, if the value you're using for the dropdown (value, not display name) a primary/unique key across the tables, then you should be able to populate these other fields in the form by matching the value chosen in the dropdown with that same value in the tables.