Hi I am a commission-only employee and my employer has me working 40 scheduled hours a week (10:30 - 6:30) during which I must be at my desk working and 1 saturday a month (10-2) on top of that. Can an employer institute mandatory hours for an employee to be in an office at their desk, and not be compensated except for their sales commission?

Short answer, yes. Being commissioned-only does not mean you can set your own hours.

And there is nothing magic about 40 hours, either, except that this is where the government requires overtime to be paid. ANY employee, regardless of how he is paid, can be required to work more than 40 hours in a week unless he has a bona fide contract that says otherwise; the only question is whether he has to be paid OT for that time.