There is an option inside of the app that allows you to upgrade via PayPal. All you have to do is:
- Open up the app
- Click the "Upgrade to Pro" item at the bottom of the Info tab
- Press the Menu button
- Click "Buy License (PayPal)"
This will open up a url that will allow you to purchase a key directly from PayPal.
If you would rather do this on a computer, here is the direct link. Once the confirmation has been received, you will receive a license key and the import instructions sent to the PayPal email address.