When GIMP is installed and ran for the first time, it will create folders to store scripts, plugins, patterns, brushes and other resources you would like to add.
In Linux, GIMP will create a hidden folder under your username. The name of the hidden folder will reflect the version of GIMP you have installed. For example, if you are using GIMP 2.8.x, the folder will be named: .gimp-2.8. Under this folder, GIMP will create separate folders for you to store scripts, plug-ins and other resources you wish to add to GIMP.
Note: you will need configure your file browser to show hidden files to be able to see these folders.
Assuming you are using GIMP 2.8.x, scripts ending with the extension .scm should be placed in...
Python plug-ins ending in .py (as well as other binary plug-ins) should be placed in...
Ensure you make all plug-ins "executable" by selecting the plug-in from your file browser, click properties and check the box "Allow executable" under the permissions tab.
After a new script, plug-in or other resource is added to a folder, you'll need to refresh GIMP for that resource or simply restart GIMP. The new script or plug-in will now appear in one of the GIMP menus. The location of the script's menu entry will vary so check the script's documentation to locate the new menu item.