Per IRS Publication 550, a Mark-to-market election for traders must be made also:

1. Go to the Other section and then go to the screen, Elections (Misc. Elections).
2. Scroll to the bottom of the screen and look for the area Election to Use Mark-to-Market Method of Accounting
3. Check the box for Mark-to-market election for traders [475(f)].
4. Enter the trade or business in the field Trade or business for which election is made.

Where in proseries is the misc election forms? I use the basic program.