You need to be in Advanced View Mode to be able to use affiliate contract feature.
After you make sure that you are in Advanced View Mode, go to "My Affiliates" page (reports >> affiliate/partner >> my affiliates):
Search for your affiliate, then under "Aff Contract" column, click the "Contract" link.
After you clicked the "Contract" link then you will be directed to that specific affiliate contract page where you can setup what kind of contract that you want to give.
You can also watch this tutorial video for more details: