In LW layout, open the Surface Editor from the toolbar (or F5 from keyboard).
To change an image for a monitor, select the "surface name" for the monitor, such as "screen_surface1". Then click on the "T" button next to the "Color" value, which opens the "Texture Editor." In the middle panel of the "Texture Editor" locate the drop down button next to the "Image" label and click it. From the dropdown list (at the very bottom) select "load image" and find the image or video that you want to display. Finally, you may need to click the "Automatic Sizing" button in the bottom panel to fit your image to the screen. Now click the "Use Texture" button. You should now see your image/video on the monitor. Repeat these steps for each monitor surface.
For the dancing girl, locate the surface named "GirlPoly" and follow the above for the "T" next to "Color." In addition, notice that the "T" button next to "Transparency" is also highlighted. Clicking this button again opens the "Texture Editor" but this time you follow the above to add your alpha mask.
Finally, when you save your scene be sure to also File>Save>Save All Objects before you close lw. LW saves surface changes with the object file, not the scene file, so both files must be saved.
The dancing girl in the 3D Compositing scenes are a special case. You need to replace 2 video clips. One is the color clip and the other is alpha channel.
You need to key out the green in your greenscreen footage and then export the sequence. We recommend 32 bit TGA files. Once you have that, go to the image editor and select the first sequence named rachel on the left and then click the replace button on the top of the image editor. Navigate to where you saved the chroma key sequence of frames. repeat that for the second copy of the rachel clip and you should be all set.
There are tutorials on the DVD that describe the process of mapping video, make sure you watch them!