To see what sweepstakes you can spend your miles on please log into the Fleet Feet app. Click on the present icon with the word "Rewards" below it, which is located in the bottom middle of the app. Then click on the "Available" section.
Here you will see items you can spend your miles on, as well as sweepstakes you can spend miles on to enter. To apply to a sweepstake, simply click on the sweepstake you would like to enter, and press the orange "Redeem" button. This will take away the correct amount of miles, and enter you in the sweepstakes.