I have a Mac and I use iTunes for my music. At this time I don’t believe there’s any way to use mood tags with iTunes.
At best, what you could do is use a Picard script that combines genres with moods and then make a smart playlist in iTunes that looks for the mood tag. That would make your genre field look something like this:
Synthpop; Pop; Easy Listening; Chillout; Smooth; Female Vocals; Melancholy: Rainy Days
Unfortunately, iTunes will read this as one big long tag, so the Genres view ends up being useless. I only use smart playlists though, not the genre view, so that doesn’t matter to me.