Hi, is this step-by-step guide helpful?
viewtopic.php?f=147&t=27158#p250799
If you have installed GME and you have a desktop shortcut, you can use the steps from the above link to set up GME. If you follow the steps, you will have a new subfolder called 'MODS' to put any mods in, something like 'Slitherine\Panzer Corps\Panzer Corps\MODS'
If you downloaded the USMC mod by bebro, you'll have a 148Mb zip file called 'USMC_GME_v0_95.zip'.
Just put this zip file in that 'MODS' subfolder and unzip it. Usually, in Windows you can right-click the file and in the pop-up menu there is an option called 'Extract all', select this.
You will get a new window where it will ask where you want to put the files, it should already show something like 'Slitherine\Panzer Corps\Panzer Corps\MODS\USMC_GME_v0_95', click the 'Extract' button and let it run.
When it's done unpacking, start up GME again and follow the steps from the post I linked at the beginning to activate the mod. The mod's name is 'USMC_GME_v0_95'. If you have any trouble with specific steps just let me know at what step it went wrong. It can be a bit cumbersome the first time, don't give up, even I managed it!
A quick reminder for in GME: if the left field is empty, let GME search for mods (select 'Tasks', than 'refresh list'). In the left field select the 'USMC_GME_v0_95' mod, and activate it with the top button in the middle.