How to set up data entry in many-to-many database in Access 2010 with forms/subforms?
The general idea is that you create a subform that's bound to your link table, and make one of the fields in the subform a combo box bound to the child record ID that's in your link table. Fill the combo box using records in the child table.

Then you handle the NotInList event on the combo box to prompt the user to add the record to the child table (or whatever is appropriate for your application).

NotInList Event

Then if you want to actually remove rows from the child table when they are no longer referenced by the link table, handle the appropriate deletion event on your subform, and check for unreferenced records.

I've implemented something like this once, but I think it was in Access 2000 on a machine I don't have with me at the moment. I'll try to check it later out of curiosity.