The cartridge is held in with the little clip you can see in the slot/hole, pull that out. If it falls into the wall you can purchase a replacment at a hardware store. The cartridge comes out with great difficulty, the hardware store will also carry a tool that makes removal easier. The cartridge is also at the hardware store. Moen will send you a replacement cartridge if you are the original owner of the home.

Turn the water off before you remove the cartridge, its under pressure.

In the picture the cartridge clips is still in but i removed it and still nothing. Also do i buy the moen 1222 or 1225 cartridge i am replacing a bath/shower cartridge.

They come out hard that's why a tool helps. Once you get the cartridge out match it. I use heavy pliers and try the work it out. Do not distroy the inner threads that's what the tool attaches to. Spray a little oil on it that may help.

As I recall, this works for some of them (eg. 1225).

Without the special puller tool, you can release the Moen cartridge by using the white cap that comes with the new cartridge. Turn off water, remove lock pin, put the cap on the cartridge, turn a little bit with wrench to loosen it (the cap is squared end so you can do this). Then it will be much easier to remove with your pliers.

No mention of this by Moen, but it works.

http://www.homedepot.ca/product/replacement-for-moen-cartridge-tool-posi-puller/814330

purchase the above from home depot. its a cartridge puller.

http://www.ehow.com/how_4925876_use-moen-cartridge-puller.html

this is how you use the puller.

to find out which cartridge you need, go the moen site and look through the pictures of faucets until you find yours and then click on the picture and all the information will be there, which cartridge you need. If you call Moen they will send you the cartridge for free. They have lifetime warranties on their cartridges. We just went through this process with mother's shower/tub faucet. It just takes a bit of patience :) Good LUCK!