To collect donations you will want to 'Add a Sale Item,' pictured below.

You can find this by visiting your Organizer Dashboard and selecting Registration, followed by Retail. Add a Sale Item (in the top right corner) for each donation amount you would like to make available to athletes as they register for your event.

By creating a different "item" for each donation amount you would like to collect (e.g. $5, $10, $20, $50) registrants will then have the ability to select the donation amount or amounts they want to contribute at checkout.