Nope, diff is separate from CVS (although many CVS clients have the functionality built-in).
Here's an introductory article:
Diff and patch go together, although diff will be more important in your case. There is a Windows version of both utilities:
It is good to know how to use both because it is nice to apply your patches before submitting them to be sure they work (as long as the version of the file being patched hasn't changed there isn't much to go wrong, but I *have* seen them fail for various reasons).
What you'll need to do is check out the code from CVS HEAD (which is always the latest), make your changes, then use diff to generate a patch and then submit it. I assume your using Windows, in which case I suggest installing TortoiseCVS:
That's what I use, and it takes virtually all of the complexity out of using CVS (once you get connected to the repository, that can sometimes be tricky for first-timers).
By the way, I just realized there is no link from the PF page (url]http://pocketfrog.sourceforge.net[/url]) to the main SF page, I'll have to add one later, so here is the URL everyone will probably want:
Once there, you should see a link for Patches, and that is where you will be able to submit them. When you submit them, please eMail me so I don't miss them
Let me know if you have any other questions. Thanks!
...and so I said to Mr. Gates: "$640 billion should be enough for anyone!"