Here are my latest Xmodap settings that I use with my iMac and Wireless Apple (UK) Wireless keyboard for running Centos 5.x under Parallels Desktop 7 (on OS X Lion):
keycode 11 = 2 at twosuperior oneeighth twosuperior oneeighth keycode 94 = 0x60 0x7e keycode 48 = apostrophe quotedbl dead_circumflex dead_caron dead_circumflex dead_caron keycode 49 = numbersign plusminus
You can place these in your home directory, e.g. $HOME/.xmodmap, or you could place them in something like /etc/X11/Xmodmap and add the following line to /etc/X11/xinitrc/Xsession
[ -f /etc/X11/Xmodmap ] && xmodmap /etc/X11/Xmodmap
The key mappings have been tested on Centos 5.7 and 6.1, although making it global, as in the line above, on seems to work on Centos 5.7.