In the Alumni.Fund tab of Site Settings, select the Checking button and enter your checking account number. Next, enter your bank account routing number. You can find these two numbers via your bank, or by looking at a check for the account you're trying to add.
If looking at a check, here is a guide to find each set of numbers:
After you've entered each set of numbers, click the blue "Update Settings" button. You'll see a green bar confirming the addition of the numbers, your full routing number and the last four digits of your checking account number.
Enabling Alumni.Fund and selecting Checking: