To accept donations on your event, click on the event and go to Tickets > Add/Edit. Create your donation ticket type. (See also: Basic Event Setup)
Click "Expand all options."
In the Capacity column, check the box labelled "Donation," and Save.
By default, the donate option appears alongside the tickets in the order process. If you want it on a separate page, after following the steps above, you can also check this box and Save.
If you want to show customers your progress toward a donation goal, enable the donation-tracker here. Enter the desired goal size and click Save.