To know the solution would require that we know the name of the subform control as already suggested. The syntax for drilling down to a control on a subform is
[Forms]![Main form name]![subform control name].[Form]![control name on subform]
You don't need the brackets unless an object name contains spaces. If you don't want to drill down to a control, then truncate that which you don't need from the example syntax.
You might need to put the main form in design view to get the subform control name. Sometimes it is the same name as the subform; especially if you let Access do this for you. Sometimes it is Child0 (or some other number). Figure that out and you should be ok.
EDIT - whenever Access gives an object a meaningless name I always change it to something meaningful if I'm not just playing around.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.