OK, here is my tutorial on it:
-Firstly, D-Rex is the rebug firmware that you install over a console that is on DEX already
-To get the godmode class you need someone on DEX to do it on your account, or transfer to DEX yourself.
- I would recommend highly as would the whole of NGU, to use this guide to change to DEX:
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-After this download the ProDG Target manager and the mw3 mod pack from this thread:
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-Install ProDG and connect to your ps3, this thread is very helpful:
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-Then in the mod pack there is a godmode class tool under the tools section and run this whilst on mw3. Then press connection and then invisible class.
-That's it! Hope I helped and if you need any more info please add me on skype: MrDaBlobz