Automatic installation with an extension key
You can get the extension key from magentocommerce webpage here.
Note: You have to be logged in (and have an account) on magentocommerce webpage
If you are logged in, click on the big orange button Get Extension Key.
You have to agree with the extension license (GPLv3) to continue to get key.
Then click the GET EXTENSION KEY button. You will be able to select and copy the key to clipboard.
Copy the key to you clipboard and go to your Magento connect. Paste the plugin key to the field number 2 with the caption Paste extension key to install and hit Install button.
Now the extension info are loaded. Click the button Proceed to continue the installation.
You will now see a log from the installation process. The successful log should look similar to the one on the following screenshot:
When the installation is completed you can return to your Magento admin panel and you will see a new menu item Liveagent in your main menu.
Note: Be sure you use the latest LiveAgent plugin for Magento.
Another nice tutorial about installing Magento extensions is here.