The MPJ file is the project file. To share your video, you should click
File >> Make Movie and follow the instructions of the Make Movie Wizard to create a movie file. YouTube suggests that uploaded movies be in AVI format and have dimensions of 640x480 pixels, using Xvid compression.
If you do not already have one, you can download the an Xvid compressor
here. You will have to restart Video Edit Magic after installing to create Xvid-compressed files.