how to apply patch ?

I have a tutorial with several steps (A, B, C, D etc).

Between each step I add a few things lets say.

Now I made a mistake in B and correct this (e.g. I forgot several concepts).

I add them in step B of the project and create also a patch.

Now I want to apply this patch to C and D. However it did not work for me.

MPS wants to apply this patch to a jared model file in the MPS installation directory.

What am I doing wrong here ? whats the trick to make MPS work correctly with patch.

any known limitations, caveats ?

thanks,

Bernhard.