I can't open your DB because it is A2007.
Another type of approach which is what use is to have a form open which is based on the table that supports the subform and when the form opens (for a new record) the ID data from the main form record is inserted plus any defaults.
I do this for a couple of reasons. Firstly, it mush better for entering data than is the subform. Secondly, there a values going in the Many table that of displayed on all the subforms associated with the Many table. A third reason is I have subforms so there is no record selector, navigation buttons etc.
You could open the secondary form the subform although i do it from the main form.