Select the modified cars and go to Mankato raceway. Go into the pits and there will be an opening in the wall. Go into the opening and you should be in the infield.

When you chose a series, chose "Modified". Drive to the backstretch all the way. When you see the concrete, stop in front of it and turn around. Forward, you will see two regular walls. Drive between them and stay on the accelerator. You should be in a different part of the infield.