There are no buttons in extensions.gnome.org website. So I am unable to download shell extensions. I have already installed gnome-shell in ubuntu 12.04. My GNOME screen looks very pale.
First you need to click on the name of the extensions in this page and an ADD button will be displayed on the left side of the extension name.
This can happen sometimes (I experienced it). Try doing these
Waiting and refreshing worked for me.