First you need a hack and second you need an injector to inject the hack into combat arms.
Choose and download a newer hack like this http://www.mpgh.net/forum/164-combat...1-24-10-a.html
Now choose and download an injector like this http://www.mpgh.net/forum/164-combat...t-1-7-0-a.html
These should be in .rar files. To extract them simply use this program WinRAR archiver, a powerful tool to process RAR and ZIP files
After you've downloaded and installed winrar. You can extract both hack and injector using right click.
1) Double click your injector.
2) Click combat arms option and engine should pop up in the text box.
3) Click browse and find your extracted hack which should be in .dll format.
4) The hack's location should pop up in the text box.
5) It should be done, click inject (although its most likely auto-inject so don't worry if an error comes up)
6) Now open combat arms and the hack will inject into the combat arm's engine.
7) Hack and start pwning.