The Preference Panel for Acorn is located under Acorn ▸ Preferences . There are three tabs labeled General, Advanced, and Registration.
The General tab includes preferences for Acorn such as turning on and off the startup window menu and setting your canvas origin to top left. There is also the option to change the hot key for taking a screenshot in Acorn, the option to turn screenshot layers on and off, and the choice of ruler units (pixels, centimeters, or inches). If you would like to use 10.7's autosave and versions feature, then check the 'enable autosave and versions' box.
The Advanced tab gives you options to change the background color of full screen mode, and to set the speed of the Precision Brush. It also includes preferences on using the image size for paper size when printing, making new layers for text boxes, hiding cursor tooltips, and ignoring trackpad gestures (applies to Macbooks only). You have the option to change the color space for new images and to automatically check for updates when you open Acorn.
This is where you enter your name and registration number for Acorn. Once your registration number is entered, it will be saved here for future reference. There is a link to the Flying Meat store where you can purchase upgrades and other software, and a link to the support page where you can request lost registration numbers. The registration tab is not present in the Mac App Store version of Acorn.
For more see Hidden Prefs.