Mylyn comes preinstalled with the most recent versions of Eclipse, so you may just need to select it by navigating to Window | Show View | Other, then selecting the Builds component from the Mylyn category like this:
Now you just need to configure Mylyn using the following details:
If you use a version of Eclipse that does not come bundled with Mylyn, you can download and install it using the standard Eclipse installation process by selecting Help | Software Updates… then add a new update site with this URL: http://download.eclipse.org/tools/mylyn/update/e3.4 (or a later version, if available and preferred).
Once this has been done, select the new update site that you just created and add the Mylyn components you would like to install.