I was having problems because I was looking to install the Maven plugin on MuleStudio not Eclipse..
[for MuleStudio 1.2 or below do steps (1) and (2) otherwise jump to step (2)]
Instructions for MuleStudio (ONLY versions 1.2 and below): (1) Help >install new software...
Helios Update Site - http://download.eclipse.org/releases/helios/
Instructions for MuleStudio (1.3) OR Eclipse: (2) Help >install new software...
Maven - URL: http://download.eclipse.org/technology/m2e/releases