how to run the command mvn eclipse:eclipse

Right click on the project

->Run As --> Run configurations.

Then select Maven Build

Then click new button to create a configuration of the selected type. Click on Browse workspace (now is Workspace...) then select your project and in goals specify eclipse:eclipse

The m2e plugin uses it's own distribution of Maven, packaged with the plugin.

In order to use Maven from command line, you need to have it installed as a standalone application. Here is an instruction explaining how to do it in Windows

Once Maven is properly installed (i.e. be sure that MAVEN_HOME, JAVA_HOME and PATH variables are set correctly): you must run mvn eclipse:eclipse from the directory containing the pom.xml.

I don't think one needs it any more. The latest versions of Eclipse have Maven plugin enabled. So you will just need to import a Maven project into Eclipse and no more as an existing project. Eclipse will create the needed .project, .settings, .classpath files based on your pom.xml and environment settings (installed Java version, etc.) . The earlier versions of Eclipse needed to have run the command mvn eclipse:eclipse which produced the same result.