Try starting the game from the actual CA launcher in the Nexon folder. If you didn't change the destination path when it was setting itself up, you should find it here:
My Computer --> C --> Nexon --> Combat Arms --> CombatArms_Direct
Click on that, and then try injecting your hacks by starting the game that way. When you click inject, make sure that the injector states that you should wait for the process 'Engine.exe'
Then go to the game launcher and press 'Game Start'
If you are still having problems, post the exact details of what happens.