STK's .mdl is a proprietary format. Currently the only tool to convert from another format into .mdl format is the Lightwave-to-STK converter available here:
http://www.agi.com/resources/faq-sys...m?FAQ_ID=11134
So if you want to use a model in STK, you'd first need to use another tool to convert it to .lwo format, and then use the converter to create the .mdl file.
If you have any other questions regarding STK, feel free to email us at
support@agi.com.