The plug-in may reside in a different menu, such as 'Tools' or 'Draw.' Sometimes, you will only see the plug-in in a context menu - the menu that appears when you right-click on something.
Be sure to read any documentation related to the plug-in to fully understand its use.
If the Plugins menu is missing, that means that you have no active plugins that need that menu item. To test, open SketchUp and click Window > Extension Warehouse. Search for the [ Example Ruby Scripts ], and install that plugin. You'll need a Google Account to download and install plugins from the Extension Warehouse. To create a new Google Account, feel free to visit this page. You don't need to have/create a Gmail email address to create a new Google Account. You can use any email address, but be sure to select the option "I prefer to use my current email address".