I cannot change my default file association.

I have a file extension "psp" for a program called "Paint Shop Pro". I go to Control Panel>Default Programs>Set Associations. I select "psp" file extension. The current default is "unknown". I click "change program. I do not see "Paint Shop Pro" in the recommended programs so I click "other programs" and it is not there. I then click "browse" and choose "Paint Shop Pro" from my program files. When the "open with" window reappears the "OK" button is greyed out and "Paint Shop Pro" was not added to the list of other programs. I recently used a register cleaner and I believe that is what caused the problem. What can I do now to make my computer recognize the file associtation that I want to choose?

Did you double click on the Paint Shop Pro .exe in the program files. Then it should pick it up.

Yes, I double clicked.

At first I did not have the "open with" option when I right clicked a file in windows explorer.
So, I downloaded a tool to restore that option from here:
Enable_Context_Menu.reg http://www.vistax64.com/tutorials/140508-context-menu-enable-disable.html

Then I downloaded the "OpenWithAdd" tool from here:
Registering programs with the "Open With" dialog

Then I downloaded the Unassociate file types from here:
A Utility to Unassociate File Types in Windows 7 and Vista - The Winhelponline Blog

Then I could change my default file association.....except for a program called "Gimp". I do not have the "xcf" extension in my list of file extensions.

I reinstalled the "Gimp" program and now all is well........