Usually when chroma keying, you'll render with a very pure and bright blue or green background (although any colour that won't clash with the stuff you want to keep will do), so change the background colour on your Lightwave render.
Next, once you've got both clips into Premiere, you'll simply put your background plate into the video1 track, your animated overlay into the video2 track above it. Now, right click the video2 track and select Video Options\Transparency from the popup menu. From here, set a Key Type of Chroma, use the colour picker to select your keyed out colour and tweak the rest of the settings to taste.
If performing the composite in this manner, remember to render out to a sequence of uncompressed stills (such as TIF or TGA) from Lightwave rather than an AVI otherwise you'll start to introduce compression artifcats which will ruin the composite.
It is also possible to perform this composite directly in Lightwave - Take your captured (real) video adn load it into Lightwave's Image Editor. Then, from the Scene/Compositing button set the background plate to be the original video.
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