1. Unzip Folder
   The first thing to do, and it might sound a bit obvious, is to unzip the file you just got  ?
2. Copy Files
   Place the “cosmos” folder in Maya’s script directory. Normally it’s here:
   Windows:  C:\Users\USERNAME\Documents\maya\scripts\
   Mac:           /Users/USERNAME/Library/Preferences/Autodesk/maya/scripts/
3.  Add start-command
   Add the following line to your userSetup.py in the “scripts” directory:
   import cosmos
   (If it doesn’t exist, use the one from the unzipped folder, or create a new blank text file with the above text)
   If you already have Maya open, you would need to restart it or run the previous code in the script-editor.
4. You are ready to roll!
   Next time you start Maya you should be able to activate Cosmos with the shortcut:
   Windows: CTRL + TAB
   Mac: SHIFT + TAB

Ps. You can of course add Cosmos to whatever shortcut you want with the following code in the hotkey editor:
cosmos.start()