You only get a 'Plugins menu' [which I suspect you are referring to obliquely] IF there are scripts [.rb or .rbs files] in the Plugins folder that are auto-loaded when Sketchup starts up... and which make menu items in that very menu.
So to have 'plugin' tools available to you, then you must first install them in the Plugins folder and restart Sketchup, and these files code must also add items to the Plugins menu [note that some scripts add tools into other menus, or context-menu, or even just make a toolbar - so having scripts in that folder is not a guarantee that the equivalent menu will appear...]
What Plugins do you have?
Where are they?
What do their instructions say above how to activate them etc etc...
😕