I don't know whether it is going to work, but try the following:
M-x customize-apropos RET
This will bring two customisable options:
This is how I have set them
The most important, I think, is the one I circled in red. If you click on the button
value menu it should give you the option to put Skim.
Remember to save your options by clicking on
set for current session and
set for future sessions, on the top of this same screen. If it doesn't work, try re-starting Emacs (after having saved the options, of course).
EDIT: If you can't find it using the
customize-apropos command, you can use the menu as shown here:
Then scroll down until you find the options I mentioned above, i.e.
tex-view-program-list, and change then accordingly.