Unzip the download. You should now have a plugin called 'LR2GMail.lrplugin'. If your unzip program didn't keep this parent directory, then you should create it and copy your unzipped files into it.
The plugin is installed via the Lightroom plugin manager.
Note: Users of Windows XP may need to install the .NET 4 framework (if it isn't already installed)
If you have donated towards the project you should unlock the trial version using the registration code that you should have received.