STLzip itself can not extract STL files.

This freeware program can only compress STL files. This software is mainly meant to reduce the upload time of your STL-files to someone who has Magics RP in his possession (for example: a rapid prototyping service bureau)

You can view the MGX files with Magics Communicator (shareware version). You need Magics RP (version 5.3 or higher) to decompress the MGX-file into STL-files.