Access help has the following information about navigation between main and sub forms.
To move from the last field in the main form to the first field in the subform, press TAB.
To move from the first field in the subform back to the last field in the main form, press CTRL+SHIFT+TAB.
To move from the last field in the subform to the next field on the main form or to the first field in the next record or next page of the main form, press CTRL+TAB. If the Cycle property of the form is set to Current Record or Current Page, you must use the navigation buttons or other methods provided on the form to move to the next record.
Although the navigation instructions provided are helpful it is more intuitive to use the TAB key to navigate. Microsoft provided an article explaining how to code a solution that allows users to navigate between form and subform with TAB and Enter keys. The articles contain links to sample downloads.