I'm having trouble making an eclipse plugin. This is what I have done so far, this create a menu item in project explorer (when right clicked) but it is disabled and I don't know why, maybe because the button doesn't do anything right now:

   <extension point="org.eclipse.ui.menus">

               tooltip="Get files"
               style = "push">

I want that when the button files is pressed then a window displays hello world. This is just something easy I'm trying to do but I don't know how to connect my java class to my plugin.xml file. Does anybody have an idea on how to do this?