Removed screws and pin for cartridge shower, yet cannot remove main piece. It is 15yr old

Clicking on;http://www.moen.com/consumersupport/installation/moen0133/moen0133.cfm
Will get you to a installation guide for replacing a Moen cartridge. You can get a new cartridge from Moen for free call them at 800-289-6636, For those stubborn cartridges Moen makes a cartridge puller. (see image) tat can be purchased at any plumbing Supply House. Although a bit pricy at over $35 you may be able to rent one if needed. Good luck, Tom

I agree, sometimes a puller is needed. You also can get tutorials from Moen Installation Help / Animated Tutorials for Moen Faucet (http://www.moen.com/consumer-support/installation-help;jsessionid=E67498E57BC05CA11FEFD25BC275F7A5)