When I try to extend the demo of GraphPad Prism to use it for 15 more days, it gives the message, "invalid demo extension code".

 Some things to check:

• Is it possible that you obtained a demo for an older, obsolete, version of Prism? The extension code will only work on the current version.
• Is it possible that you are not entering it fully? Be sure to enter the hyphens.
• Make sure you have not entered any spaces before, or after, the code.  It is easy to include a space or two when you copy the extension code to the computer's clipboard, and then paste those spaces as well as the code. Prism (up to 5.01 and 5.0a) gets confused by the extra spaces.