Sometimes, a person wants to register, but instead of going the registration process, makes a donation by mistake. There is no way to convert a donation into a registration since the registrant did not choose a ticket, enter a user name and password, his/her goal, or answer any of the custom participant questions.
So that this person can register properly without paying again, you will need to create a 100% off promotion code, if you don't already have one. Directions to do so can be found here: Wizathon: Setting Up a Discount Code
Send this person the promotional code to be used on the final check out screen, and request that they register properly.
After the registration takes place, you, as the administrator, can apply the original donation towards the participant. The instructions on how to do this can be found here: Wizathon: Transferring a Sponsorship Amount to Another Participant/Team/Event