You have downloaded an RBZ file - a type of ZIP file - containing the files and folders needed by the plugin [aka an 'extension'].
It needs to be 'installed'.
SketchUp menu: Window > Extension Manager > Install Extension... [red button]
Find the RBZ and install it.
Most installed extensions are then immediately available in their [sub]menu items or toolbars - read the usage details for each one.
BUT, in the case of most of Fredo's tools [and many of thomthom's and a few other authors] you must also install a set of helper files - in this case:
https://sketchucation.com/pluginstore?pln=LibFredo6
After installing the Lib, you are probably best advised to restart SketchUp to ensure everything is synced.
You only need to install the Lib once.
Unless of course a new version is available, and a particular extensions says it's installation is necessary.