What do I need to do that I can access google calendar on thunderbird?

For Ubuntu 10.10+

Installing the necessary components is just a matter of running:

sudo apt-get install xul-ext-lightning xul-ext-gdata-provider

This will install the Lightning Calendar extension and the Google Calendar provider.

For Ubuntu 10.04

Unfortunately, Lightning is not available in the Ubuntu repositories for 10.04 LTS users. If you are running Ubuntu 10.04 LTS, you have 2 options:

1. 32-bit users can install a version of Lightning compatible with Thunderbird 3.1 from https://addons.mozilla.org/en-US/thunderbird/addon/lightning/versions/?page=1#version-1.0b2
2. You can install the latest stable version of Thunderbird, following the instructions in How do I install the latest stable version of Thunderbird?. The thunderbird-stable PPA also has a compatible version of the Lightning extension for Ubuntu 10.04 LTS users.

I would strongly discourage disabling the addons compatibility checker, as advised in another answer. The check is there for a very good reason, and disabling it could make Thunderbird unusable.

I just tried it out, and you'll need

1. Lightning
2. Provider for Google Calendar

To disable compatibility checking for extensions, open Thunderbird's preferences, switch to the "Advanced" tab and in the "General" subtab, start the config editor. (source)

Add a new boolean value via right click named

extensions.CompatibilityCheck.[VERSION]

set to false.

[VERSION] stands for your Thunderbird version, e.g. 3.1, if you're using 3.1 stable, 3.1a if you're using 3.1 alpha, or 3.1b if you're using 3.1 beta. (source)

Then you can just install your extensions.

Should the Config Editor be unavailable for any reason whatsoever, you can always manually edit ~/.mozilla-thunderbird/[somecharacters].default/prefs.js.

Add the following lines as necessary:

user_pref("extensions.checkCompatibility", false);
user_pref("extensions.checkCompatibility.3.0", false);

You need the 'Lightning' extension for Thunderbird (http://www.mozilla.org/projects/calendar/) and the google provider plugin (https://addons.mozilla.org/en-US/thunderbird/addon/4631/).

You can then ask the creation of a new "network" calendar, and identify it using the "private URL" that Google Calendar gives you (in the calendar's preferences).