The easy way is click on the note in the upper left that says "You have $0.00 in DonationCredits":

Go through Paypal to get some credits. Then you can donate by clicking the coin under the person's name:

The value is up to you! How much do you appreciate what was done for you? How much effort will this save you? If you bought this from a store what would it cost you? Think about those guidelines, just remember it is a token of your gratitude. I think a lot of folks here get real enjoyment for writing a program to fulfill a need, but maybe a cup or two of joe would make their day nicer!