On the Mac hit the option and the accent, or the character that has the accent on its map (say, option e for a ´ ) then hit the character you want the accent over.
So é is option e then e
And è is option ` and e
The tilde is a slightly trickier one as I don't think there's an automatic kerning character available for it so you either need to hit the ~ then the character and kern them together (probably with some baseline shift on the ~) or you need to look up the specific pre-accented character (and o with a tilde over it is available in most faces). That's not too hard if you have Quark 7 as it now has a glyphs palette.
Hope that helps!
And being a pedant - ~ is called a tilde. Tilda is a brand of rice!
:D
Greg
I think therefore I am, you're pink therefore you're spam