To enable donations:
At Setup > Settings, select Fees and Payments and check "Allow Donations" and click Save.
When donations are enabled, a "Donation" field will be included In the order form.
The amount inserted in this filed will be added to the total of the order.
Optionally, you can add a new ticket type (with a name "Donation" for example) with price 0. When someone selects it, he can enter an amount in the "Donation" field.