Can someone tell me what I’m doing wrong? I have everything setup correctly as far as I know, but it won’t render correctly. Each time I try to render, a black screen comes up.
You’ve got ‘do sequence’ switched on, which will render the contents of the sequence editor. try switching that off.
Try putting the alpha from the movie to the bottom input of the alpha over and the render in the top one. SOmetimes that has to do w/ it. Also instead of using alpha over use Space bar>Color>Mix. Now switch that to a add. Try both. And try what the pro(broken) said. 
I tried what broken said, and it still didn’t work. When I press render, a window comes up with the monkey composited and everything, but it still doesn’t render. It seems to be stuck on the first frame.
Make sure you are pressing Anim or Control F12. Also, if you want a video instead of a sequence of images, you should set the Format menu in Format Panel to a video format (avi, quicktime, ffmpeg). Lastly, if you want to dictate where the animation is going to be saved, you have to set the first path bar in the Output Panel.
from here it looks like you want to use the 1st 228 frames of the video (frs: 228 Offs: 0) but you want the clip animation to start at frame 371 (SFra: 371)… since you are only compositing from 1-131, you are only going to see the 1st frame of the clip.
maybe Frs: 228 SFra:1 Offs:371 is what you wanted…
nothing is working. . I don’t understand. . I’ve done this before. I followed this method the first time and it worked perfectly. .
In the Buttons Window go to Scenes (F10). In the render tab use Premul instead of sky, In the Anim tab press Do Composite, and make sure that in the Format tab all the settings there are the same as your source video. Go into the Node editor and click on the Composite Nodes button and then Use Nodes. Add an image( spacebar > input > add > image) and click the up and down arrows and load your video. You might have to manually set how many frames there are but you can just set it to a ridiculously high number that will cover it. Also click on the icon on the bottom right of the node that looks like a car. Add an Alphaover Node ( spacebar > add > color > alphaover). Attach your source video to the top image input on the alphaover node and attach your rendered layer to the bottom one. Attach the alphaover node to the composite node and that should do it.Then, click do sequence and choose your render settings. Then render.