Using the plugin system in your addon is fairly simple. The most important part is good planning because, to some degree, you're defining an interface for other people to use.
To activate the plugins for your addon, you just have to use the following line of code:
$result = $mainframe->triggerEvent( 'onSomething', $param );
This function calls every plugin that has registered itself for the event 'onSomething' and hands over the array $param as parameter.
As a return value you get an array of results from these plugins. All this is of course optional, you could also use the system without parameters and/or a result.