How to Register Acorn
Once you have your registration number for Acorn, you will want to plug it in so Acorn knows you have done the good deed and that pesky watermark goes away.
To do this, choose Acorn ▸ Preferences ▸ Registration from the menu bar.
A panel will appear, with fields to enter your registration name and number. Make sure when you enter them they are spelled exactly the same way as given to you when you completed your order (either via email or the web page). It is recommended that you cut and paste into these fields.
Then press the "Register" button and you are good to go.
If you have already purchased Acorn and you do not know what your registration number is, visit Flying Meat's support page and we will send you an email with the information: http://www.flyingmeat.com/support/.
Mac App Store Version
The Mac App Store does not provide a registration number for Acorn, since the store manages all the applications you purchase through it for you. You will notice in Acorn preferences that there is no registration tab if you bought Acorn on the App Store.
If you installed the trial version of Acorn prior to purchasing on the Mac App Store, you will need to delete the trial version of Acorn from your computer. Perform a spotlight search for "acorn" (without the quotes). Delete all copies of Acorn. Install or re-install Acorn from the Mac App Store. It should be listed under your 'Purchases' tab. If you clicked the 'Install' button and Acorn isn't showing up in your applications folder once it has completed installing, try re-booting.