Plugins need installing into the local Maven repository before use.
After you have downloaded a plugin for Maven (from Maven-Plugins, or from a different source), you should look in your Maven installation and you will find $MAVEN_HOME/plugins/. You need to copy your plugin into that directory. If you then type maven -g you will get a list of all plugins and goals that Maven can find (note that you may need to remove your ".maven/cache" directory first, because Maven caches plugin information).
You can set up your Maven installation to "know about" the Maven-Plugins project. To do this you need
http://maven-plugins.sourceforge.net/repository to your maven.repo.remote property
(can be set in project.properties for example).
Typically it will look like
You can then download and install plugins as follows
maven -DartifactId=ARTIFACTID -DgroupId=GROUPID -Dversion=VERSION plugin:download
maven -DartifactId=maven-findbugs-plugin -DgroupId=maven-plugins -Dversion=1.0-SNAPSHOT plugin:download -Dmaven.repo.remote=http://www.ibiblio.org/maven,http://maven-plugins.sourceforge.net/repository
Make your project aware of the SourceForge repository by following the instructions above.
Instead of manually downloading, you can add a dependency in the project.xml file so that anyone building your product will have the plugin automatically downloaded and installed.
<dependency> <groupId>maven-plugins</groupId> <artifactId>maven-(PLUGIN_NAME)-plugin</artifactId> <version>(PLUGIN_VERSION)</version> <type>plugin</type> </dependency>