First of all, make sure you have the WinRar. I think you can't open it because you don't have it. It will open the .rar file.
You can download it here:
Go back to the original post and look at the instructions. If there aren't any instructions, extract it to your desktop, or wherever it is easy to access.
Find a the .exe file and double click it. That's your injector. If there's a .dll file included in the .rar file (the original, downloaded one), I think you have to put the injector (.exe one) and the .dll in the same folder. But usually, there won't be, for noob-friendly injectors.
Please correct my errors if i'm wrong because I'm not 100% sure of this but that's what I did
Please press thanks if I helped!