Change your query so that instead of the IIF being in the criteria, put it in the select. And check the True/False field of the table.
So you would have a column like this:
Then in the criteria, simply reference Forms![FormName]![OptMHs]
So what this is doing, is if you want both by selecting All Man hours on the on the form, IIF will return 3 in the ManHrstoReturn column so 3 = 3.
If you don’t select All Man Hours on the form, IIF will return either 1 or 2 based on if the true/false column is true or false. And the criteia based on what option was chosen will filter the records.