The Google Plugin for Eclipse depends on other specific Eclipse components, such as WST. Your installation of Eclipse may not yet include all of them, but they can be easily installed by following these instructions. Eclipse 3.7 (Indigo)
Select Help > Install New Software...
Click the link for Available Software Sites.
Ensure there is an update site named Indigo.
If this is not present, click Add... and
enter http://download.eclipse.org/releases/indigo for the Location.
Now go through the installation steps; Eclipse should download and install
the plugin's dependencies.