Try something like this:
=iif([commselect]=1,sum([commission]))
That should do the trick...assuming that you are putting this in the footer of a grouping on the report, so it gives you the sum of commission in that grouping. If you need something else (the sum of all commissions, the sum of all commissions where commselect is 1, etc) let us know, I couldn't tell from the way you described it. Hope that helps.
Kevin