I don't know whether it is going to work, but try the following:
M-x customize-apropos RET
then write:
tex-view-program RET
This will bring two customisable options:
tex-view-program-selection and tex-view-program-list.
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-selection and tex-view-program-list, and change then accordingly.