Is the plugin in the right folder of blender?
Is the model selected, in object mode, in blender?

Hope, this helps.
I exported with the exporter from blender to MDL7 several times, I can assure you that it works!

EDIT:

This is the path where you have to place the file:

C:\ProgramFiles\Blender Foundation\Blender\.blender\scripts

There are two pplugins from ventilator:

export_3dgs_mdl7.py

and

export_gs_mdl7_static.py