To use LR/GMail you need to choose the images to send.
Enter your Google username (this is your email address with GMail) and password, then press connect. Doing this serves to verify your credentials, and it also allows LR/GMail to start collecting the contacts in your "My Contacts" group. The contacts will be offered as you start typing into the To, Cc or Bcc fields.
Once connected you'll be able to write your email. Press export when you're ready to send.
When typing your message you'll probably wish to press the <return> key to start a new line. Unfortunately on a PC this will not work. Instead you need to type in '\n', and this will be converted to a new line. I'd love to offer a better solution, but this is a Lightroom restriction that I can't avoid.
The "Clear field" options
The contents of all the fields in the LR/GMail panel are remembered by Lightroom when you close the export dialog, and then presented again when you reopen it. In general, this is a very convenient feature, however in the case of LR/GMail it's unlikely that you want to send exacly the same email as you did the last time that you opened the export dialog.
One solution would be to create a preset with all the fields left empty and then realod it to start a new mail, but that would be a frustrating extra step. Instead, the "Clear field" options allow you to choose which fields are left black when the export dialog is reopened.
Exporting from the export menu
If you've created a preset that uses LR/GMail, then the plugin acts intelligently when you start an export via the menu option (right-click on image -> Export). All the preset options are preserved, however the LR/GMail panel is always displayed so that you can write your email.
The plugin keeps track of which photos it's sent via GMail. You can then filter for these images using the Filter Bar.
Furthermore, the plugin also maintains a list of all the recipients of a given photo, so you can see who you've sent it to.