I'm fairly sure he uses Adobe Premiere Pro now to add the recorded facecam.

Its prity simple Bro if you go to "windows live movie maker" most computers have that as a default editing program. Once your in it should have a webcam video click that and then click the button when your ready to record.

Editing note -
When your editing you can either delete the webcam audio of you talking or the in game audio of you talking. I dont think you can change the mic recording options in windows live movie maker. Make sure you delete one of the audios or you will have 2 audio's overlaping eachother. Pewdiepie may use something else now im not sure but he used windows live movie maker. You can see in his old videos.

Apples's QuickTime Player has an option that allows you to do it, but its only available for QuickTime Pro, hope this helps

I can't get my emails up and it's important. Help !!

Yeah, I think you meant to make a thread, right?

Ive seen the ends of his videos and was confused as well. But I got it now, bro.

He uses the add on in Windows Live Movie Maker Beta for the webcam integration, but that doesn't add it to his videos.

He syncs his video recordings with his face recording using Sony Vegas. As do I. Its simply easy to use, but to get...kind of expensive. He also uses Audacity.

I have a tut for audacity if youd like to figure out how to use it

Hope that helped bro!

Freziod