You want to view the source of the 'main' page, the one that specifies the frameset?
I can't actually check that this works in Safari as well, 'cause I can't think of any frameset-driven sites to check it on; but, assuming it works the same as how I recall it working in IE on Windows, just go to View -> View source, instead of right-/option-clicking on the page.
That will only get you a short page, though, with a frameset defined in it. You can always just right-/option-click anywhere on any of the frames on the page and view the source that way, that gives you the source of the individual frame you're right-/option-clicking in.