If you have the SketchUcation PluginsStore toolset installed it's even easier than renaming the .ZIP as .RBZ ! ...
The SketchUcation submenu item 'Archive Installer...' lets you install the contents of a chosen RBZ file OR a selected ZIP file...
'Add-ons' for SketchUp are ALL actually 'Plugins' - scripts written in Ruby.
Some of them are written in such a way that they are ALSO 'Extensions' - this kind of Plugin allows you to control their activation using the clunky Preferences > Extensions options...
In a moment of madness in v2015 Trimble renamed the 'Plugins' menu as 'Extensions'.
The folder containing them is still named 'Plugins'.
This broke all imported Shortcuts from earlier versions which were looking for Plugins/.. and offered users nothing more than confusion about naming...
I can only surmise that it was during a fit of pique, since SketchUcation's toolset offered better management of activated Extensions etc, and similar management of ALL Plugins too...
Their Extensions-Warehouse only hosts 'Plugins' which are written as 'Extensions' [which they also 'vet'].
SketchUcation's PluginStore also hosts many of these in parallel, and also hosts many simpler 'Plugins' too...