1. Install Gnome from synaptic package manager,

 (gnome 1;3.0+Ubuntu1 / The GNOME desktop environment. with extra components) 

2, Go to advanced settings and make sure new extension turned on.

3. Go to website https://extensions.gnome.org/#page=1 this shows the available extensions, when you have turned on the correct extension from your advanced settings it will show you what extensions you have installed on your computer/

4. You will then need to create a user account to download the apps,

5, to download click the required app, this opens a link where you switch the extension on, a dialog box then appears, click install.

enjoy!!!   Gaz