I don't know if this is the only way, but I found the following at the indicator applet site. At this Launchpad post, they suggest following these directions to install Gmail Notifier, which worked. I followed the first suggestion, which consisted of:
sudo add-apt-repository ppa:gm-notify-maintainers/ppa sudo apt-get update && sudo apt-get install gm-notify and then System > Preferences > GMail Notifier Configuration to enter your gmail account information.
I also wanted to remove the unused evolution section of the indicator applet menu. I followed the advice from How do I add/remove an entry in the Indicator Applet in Gnome? and added the file "evolution" to the directory ~/.config/indicators/messages/applications-blacklist, which contained the entry:
/usr/share/applications/evolution.desktop Finally, to restart the gnome panel with the changes I ran
killall gnome-panel so that I didn't need to log off.