It really depends on the error you receive. It is possible that an Anti-virus program, Or Firewall that you have activated is block PerX as it is viewed by some as a threat.
If you have an anti-virus program or such running, Please add it to your exceptions list.
If not; Try another injector, A variety can be found here:
CrossFire Spammers, Injectors and Multi Tools - MPGH - MultiPlayer Game Hacking
There are no actual requirements as far as I am aware of.
If the injection process isn't working, Please follow this tutorial ;
If your hack still does not work, Then it is likely detected.