Well I finally took the car to an auto upolstry shop I know and they explained that with these pieces and all other aftermarket upgrades available for armrests, they must be glued onto the existing armrest. One cannot dissassemble the existing armrest cover and replace it with an aftermarket piece that does not have the exact same shape and dimensions as the original armrest cover. The Performance Products piece was large enough to and designed to fit over the existing cover. Oh Well.
I glued it on. It does look great -- its a beautiful piece of burl wood and matches exactly but, if you are really picky you may see traces of the old cover from the side. I could always go back to OE if I like.
C55: Evosport headers & UDP's; AMS crank; TVT thermo; phenolic spacers; Denso IK16's; EuroTech ECU; RennTech cf airbox w/ Green filters; Z06 catch can; Sprintbooster; Optima RedTop; Quaife LSD; AMG: shift paddles, 030 rear calipers & rotors; Rotera brake lines; Euroteck CF lip; "C63" rear diffuser; 18" SLK AMG Turbines (summer); 18" OZ Superleggera's (winter); K40 dual f&r radar w/ laser jammers; MB: digital climate control, iPod kit & mp3 CD changer; iTronic B/T for MOST.