I have the same problem with my Asus X58 board, i could actually see the raid details flashing up on the screen breifly just after the end of post, however it flashes up to quickly to read, but after a few times i manage to catch it and in fact it is CTRL+I, so the manual is wrong, i suggest any one with this Marvell controller tries I rather than M, if that does'nt work try other letters
ctrl-i is for the intel sata controller and ctrl-m is for the marvell controller.
I'm having a problem with this though. ctrl-i works and gets me into the intel raid screen, but ctrl-m doesn't work and that's what I want!!