Have you installed the plugin and activated its content that allows blender to use .m3 files?
When opening blender to whatever your starting file is check the following:
- In the properties panel, choose the 3rd icon from the left called Scene. if you close all tabs you should see about 20 tabs all starting with the keyword "M3". (starting under custom properties)
- when using the browser menu go into File --> Import/Export, it should read "Starcraft 2 model (.m3)" at the bottom.
When this is NOT the case, you must first install the plugin from PrintIn to actually work with the Starcraft 2 models.
you can download it here: https://github.com/stante/blendm3
Download and install the folder in the proper map of your blender installation, instuctions should be also at the link above.
Last step from there is: activate the plugin in blender.
Open blender, go to: File --> User preferences --> Addons, search on import/export and locate "import-export: importer and exporter for Blizzard's Starcraft 2 model files (.m3)" and check the box next to it.
I hope this helps for getting you started in the amazing world of modeling :)