The demo version of Prism 4 Mac worked fine for me, and I bought the full version. Now when I open my files, I can't print.

Although the demo and full versions of Prism are supposed to be identical, in fact there there is a subtle difference between how the demo and full version of Prism 4.0a Mac save the page size in the file. When the full version of Prism 4.0a opens a file created by the demo version, it can get mixed up and refuse to print.  

The simplest way to fix this is to check the option in the File&printer tab of the Preferences dialog to "Do not save (or read) page size ..." . 

If that doesn't solve the problem, try this: Create a new empty file in Prism Mac (full version). Then use File Merge to bring in the file created by the demo version. This should force Prism to reconfigure the file to print properly.