M2E connectors are a bridge between Maven and Eclipse. When a maven plugin is bound to a lifecycle phase like generate-sources, or process-sources it becomes a part of Eclipse build. The connector may decide if the plugin needs to be executed or not depending on the set of files that have changed since the last build and after the plugin processing is complete it may refresh workspace resources so that the changes become visible in the workbench. A connector may also add project natures or modify project classpath when Maven project configuration is updated.
For more information about M2E connectors, see M2E wiki
Once they are included, the connectors will be installed automatically when a project that is using respective maven plugin is first imported into the workspace. Alternatively they can be installed manually from the catalog through Eclipse Preferences > Maven > Discovery > Open Catalog.
Another method is direct installation from the P2 milestones repository, using Eclipse Install new software wizard.
If you would like to experiment with development builds, you can find most recent org.objectledge.maven.connectors:eclipse-repository artifact in oss.sonatype.org snapshots reposiotry and use it as local, archived update site in Eclipse Install new software wizard.
There is also P2 development repository but it's not updated automatically on CI builds, only manually. Watch the blog posts.