If you've recently added PayPal to your site you'll have to add it to any existing forms. (It will automatically show as an option on any NEW forms that you create)

To add it to any existing forms:
  1. Edit the donation form
  2. Go to step #3 Design Donor Screens
  3. Edit the Donation Form
  4. On the right box, you'll see a list of items that are on the form, you'll look for 'Payment Type'
  5. Select the Payment Type and click the 'Save Order and Edit Selected'
  6. Once in that step you'll look for the option to 'Allow payment with PayPal:'
  7. Check the box to 'Yes, allow donors to pay with PayPal.'
  8. Click Save.

Now that form has the option to pay with PayPal.