Be carefull not to mix up the maven-eclipse-plugin and the m2e Maven eclipse integration.
You should use the Maven eclipse integration (m2). If you have maven commands in your IDE I suppose that you have installed m2e successfully (if not check this answer). In this case you can convert the project to a Maven project and the Maven Dependencies will be added as a Library to your Build Path.
As far as I know, if you use eclipse:eclipse
the maven-eclipse-plugin will "mess up" your eclipse .classpath file and put every dependency as an own entry to the buildpath (and possibly the m2e configuration will be overwritten).
So both "tools" have different concepts that must not be used together. You have to take a decision which integration you prefer - IMHO m2e is mostly used.