Matlab is complaining when you try to save your Matlab path because you are trying to overwrite the global pathdef.m file, which is (obviously) write-protected. however, if you look at the error message, MATLAB also asks you if you want to save it somewhere else. You should make sure that you have a directory in your home directory called matlab, and then tell matlab to save your custom pathdef.m file there.
Figure 1. The Matlab dialog that prompts you to save your pathdef.m file somewhere else.
Figure 2. The Matlab dialog that shows the saving of the pathdef.m file in an alternate location.