Not sure if your using a wheel but heres a setup for RA on a G25 and some info for it. If your using a controller just program it in in the options menu. If the items are grayed out during a race it means that either the race or online host has disabled them.
Look at this thread to help with G25/G27 tips. Lots of older tips about the G25 when it was first released. Yes most is related to GT4/G25 but will help with GT5P/GT5. Also look at my G25 pictures for some diagrams for the G25.
The RA menu is just a menu for adjusting various car settings. TCS, ASM, ABS, torque split and brake balance. Bring up the RA menu (by pushing whatever button you have assigned to it), scroll to the correct menu option, then adjust it.