Simply download the Jabber Plugin, and set it up for Google's server.
Here is a post from the forums, describing how to set it up: 
Don't forget, if you already have a normal Jabber account, you can use both at the same time, by using the Multiple Accounts method.
Read more in Google's help center: "How do I configure Miranda for Google Talk?"
More information on Google Talk is available on the Google Wikia.