As you added the thread in the Plug-in-forum: It's no plug-in anymore. It's integrated in ILIAS.
As far as I see, first you have to add a top-item in Administration > Layout and Navigation > Main Menu > Top items.
Decide if it's a container (bunch of items/links) or a (single) link.
Afterwards you go to Administration > Layout and Navigation > Main Menu > Items and add an item.
Decide if it's a repository link etc., add an icon, match it with the already created Top item and set it active.
Or where exactely did you struggle?
As far as I see, first you have to add a top-item in Administration > Layout and Navigation > Main Menu > Top items.
Decide if it's a container (bunch of items/links) or a (single) link.
Afterwards you go to Administration > Layout and Navigation > Main Menu > Items and add an item.
Decide if it's a repository link etc., add an icon, match it with the already created Top item and set it active.
Or where exactely did you struggle?