What this patching does is creates an xml file of the differences between 2 RPDs. You can then edit that XML file if you so desire. Most of my duties over the last couple of years have centered around the RPD and front-end stuff, not nearly as much on the administrative side (i.e. migrations). So I need to catch up with the rest of the world.
Reading through the docs, I'm told that I need to use the "Compare repositories" dialog. OK, easy enough.

Where is my entry for it?
OK, let's try the Compare entry.

Since I'm using the prod_20120122 RPD, I select the dev_20120122 RPD.
I'm prompted for the Repository Password and then it spins for a few seconds, then gives me this

I select Yes.

OK...where's the Compare Repositories Dialog like the docs say?
I see the icons have changed signifying differences, but no dialog as mentioned in the docs.
What if I select No?

Ah, there it is.
Interestingly, if I maximize that Compare Repositories Dialog, the next time I run the Compare, I can see it plain as day.

Hopefully you'll find this next time you endeavor to learn how to patch your RPD and can't seem to find the Compare Repositories Dialog.