Ok so you have propably might have guessed the mvn2 plugin does not come with the standard distribution. So what you need to do is to add an update site.
You can do that by using Software Update feature in the Eclipse's toolbar.
In Ganemyde version you need to press "Add Site" button and enter the following update site url for mvn plugin:
This linked is provided on the official site opf the mvn2 plugin:
In fact most of the resources can be found there.
The only thing I can add to that is that if you have an error saying: Could not complete your request...
When you get that message all it means that you have not enabled the main "Ganymede" update site to pull the plugin's dependencies.
In order to do that you need to switch to avalable Sotware Tab and enable the Ganymede site by checking the box. The do the mvn plugin installation.