1) Do you have a gtalk account? For those who do not have one set one up
2) This is how to install it on your site
Visit Google Talk Chatback Badge and get the html code for your account
TIP! It helps to already be signed into your google talk account in the background
Once you have your code copied to your clip board sign into your google site, navigate to the page where you want to show you are available for chat.
Click on edit > insert > more gadgets
In the search bar type codewrapper (all one word), select codewrapper which is 2nd on the list
Paste in your code from the google talk badge page > change the width of the gadget to 200 pixels and the height to 60 pixels > untick the title box and border box > click ok
Now ... Chat, chat, chat!!!