It is rather simple.
First of course, open Sound Center and select the sound you want. Click the stars to give it the proper rating you with to give.... then, look at the very top of the Sound Center's GUI and you should see a tool bar with a bunch of icons on it. Presets, a few arrow buttons and right about centered you will see a little FLOPPY DISK symbol what will say "SAVE" when you hover your mouse cursor on it.... click the "SAVE" button and that should save the stars rating for you.
I opened my DAW, found a non-starred instrument, gave it 4 stars and saved it using that method. Shut off the DAW and reopened it with a NEW project, loaded CSC and sure enough the instrument had 4 stars on it.
Hope that answers your question.