Because you have Xbox live and MW2 can be played online?
If it's in your XBOX, you have to buy xbox live then put in the code on your xbox, and you can play live for mw2.
Yes.
You can't.
yes, you need to pay to do anything on live
yes
you cannot do this
theypaly mw2 on the xbox
huuum no
Nope.
Zombies are not on MW2 only WAW and Black Ops
No, sadly.
Never. Just because you are online does not change that the PS3 can not play a Xbox 360 game and it also can not go on Xbox Live. The games have the same title but the are not the same. They can not play each other anymore than you could decide to get the other consoles version and expect it to work on your console or either console to play the PC version of MW2.