Login to our PluginStore using the link near the top of this page.
Use its search tool to find what you want to download.
An RBZ file will download onto your computer.
You then use Sketchup's Extension Manager to install the extension from the downloaded RBZ.
Note that Fredo's extensions also need LibFredo installing - get its RBZ from the PluginStore...
Most of Fredo's extensions are now licensed [not very expensive]... to get the licensing tools you need to have the SketchUcation toolset RBZ downloaded and installed.
After the installations restart Sketchup to ensure everything syncs.
Fredo's extensions have their own menu items which allow you to buy / manage your licenses etc.

A simpler way which sidesteps the RBZ files download, is to install the SketchUcation toolset from its downloaded RBZ - there is a tool in its set called ExtensionStore³, this has all of the contents of the PluginStore and you can select an extension and click the red button, and it will be auto-installed without needing to download its RBZ manually...
It also reminds you if there's an update available etc...