It doesn't seem like Myriad font is included with Apple, which is odd because its system font is Myriad. Isn't this supposed to be included? I like it a lot but I don't want to pay $30 for it.
Apple's print/marketing font is indeed various forms of Myriad (semibold a lot) however Apple uses Lucida Grande as the system font. Adobe is creator the Myriad font collection and would cost a lot if Apple added it to the default fonts in OS X.