AGI has a conversion program that allows you to convert Lightwave (.lwo) format files to an MDL model. If you have built your model using something other than Lightwave you will have to first convert the file to .lwo format before you can convert it to an MDL model.
Some file formats can be imported directly into Lightwave and then saved or exported in the Lightwave format. Also, many 3D software packages can save or export files in the Lightwave format. Check your 3D modeling software documentation to find out if it supports these features.
There are also 3rd party utilities that will allow you to convert 3D models to the Lightwave format. They include:
Keep in mind that MDL only supports a small subset of what Lightwave is capable of rendering. This is because Lightwave can take its time to process each animation frame, sometimes taking an hour or more per frame, whereas STK will attempt to render your model in realtime, and make it interactive with the user. For this reason, it is recommended that you use far fewer polygons in MDL files than you would for a Lightwave-rendered animation.
Generally speaking, if you make your model look good in Lightwave's built-in preview window, then the model stands a good chance of converting nicely. The preview window uses OpenGL for rendering, as does STK. So, the converted model may look more like the preview window than a finished Lightwave-rendered image.