Wrong folder. Normally, you can put things in that folder, but you are not an admin, so your computer won't let you. Use the user folder instead. I THINK it is something like "~/.megaglest" though I'm not a linux users so can give no guarantees, though hopefully you should be able to find it. If not, open the INI file called "glest.ini" in the installation directory, there will be a value like "userdata_root" or something which will show what folder the user data is stored in. This is where you will place downloaded mods.