Some authors like Fredo & thomthom use supporting Lib files, which are available to download and install [LibFredo], then after SketchUp is restarted the tools should load and be available.
However, don't fall into the trap of thinking that every extension is listed under the Extensions menu...
Read up on each particular extension's usage - e.g. Fredo's Tools is accessed through the Tools menu list or its Toolbar entry...
Some other tools are also activated from the context-menu... read up on each one...