I had everything on another computer but since it got a virus I have had to get a new computer. Now I can not play my videos and I'm trying to figure out what happened. It appears I need to activate the codecs for the MPEG-4 files. How do I do that?
. . . . I need to activate the codecs for the MPEG-4 files . . . .
That depends on which version of MEP you have and if it is Windows 8 (assuming this is the version of Windows on your new computer) compatible.
Assuming the version you have is Windows 8 compatible and your MEP is the Plus or Premium versions then when you add a mp4 to the timeline the message should pop up asking for activation and gives you the option to activate online in which case you follow the message instructions / options.
IIRC the mp4 codec for the Basic version of MEP has to be purchased.
If you were still running MEP 15 as in your previous post then you will need to upgrade to a Windows 8 compatible version.