Generally speaking an AD carry who leans on abilities for AD damage (MF or Caitlyn, but not Tristana or Ashe) should emphasize +AD and put less emphasis on attack speed or +crit. The reason being, of course, that such items enhance autoattacks, but autoattacks are not your focus.
Try to build/play so you don't rely on meki/manamune. They aren't as bad as a lot of nerd ragers want to make them out to be, but late game the only damage +mana really nets you is in the rare circumstance that you're otherwise out of mana. That's not likely later. Team fights go far too quickly and your natural reserve will get you through.
This just reminded, off subject, of a bunch of nerd ragers cursing me for grabbing a meki on Karthus, even though I had done so because between that, runes, and masteries, I could lay waste almost constantly. He's a mana-dependent AP carry though, so the extra mana did literally translate into extra damage. You shouldn't have that so much with MF.