To enable donations:
At Event> 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.