Prior to pushing out the slide stop, you have to tip the ejector assembly down to free the slide stop for removal.
Please note that the slide stop WILL NOT come completely out of the frame, like many other guns;
it stops in the fully extended position, allowing disassembly, but remains in the frame.
Disassembly instructions start on page 15 of the manual (linked from Ruger's website):
I recommend paying close attention to step #3. A friend of mine didn't, and he regretted it quite a bit.
If you give it a chance, it WILL bite you -- hard.
Good luck and happy cleaning!