Your sample database does not work, I am getting the same error when I try to open sfrmActivity_List
You have a criteria in the query driving that subfrom of :
fOSUserName()
The function is not defined in your sample database, fix the criteria or add the function please. I can not guess unless your desired result is that you:
1. Choose a user from a combo box on your MAIN FORM (main form is an unbound form)
2. the list box in the subform is updated with items related to that user
3. select the item from the list box you want to edit which opens another form that relates to the item selected on your subform.