With what you've described so far...I have a bad feeling that you're going to bust something...and if not you're going to have problems putting it back together.
1. Put that little screw right above the nozzle back in. It's probably ok as it is now...but normally you'd take that out when you're ready to open the whole mechbox.
2. Lift the little stub that's in the trigger guard just in front of the trigger up and push out the pin. Can't recall exactly if you need to lift that stub, but I think you do. Some pins are very, very hard to get out...push with a large allen key if you need to.
* your selector position should be on Semi...
* I can't remember exactly (it's been a long time)...but if there is a large center screw in the bottom of the grip cap that's screwed into your motor cage you'll want to take that out. (don't think so on the G36/MP5 though....definitely on an AK)
Once that pin is out...the mechbox should lift out of the grip. You might need to tilt/wiggle it as you lift it out...'cause some wiring/selector pins might be in the way.
Then refer to mechbox.com for video's on opening mechboxes. If there isn't a V3 G36 specific one...it's very similar to any V3 mechbox...and with just a minor difference here or there very similar to a V2.
* don't smash your hopup unit putting it back together