The same way you install any other Extensions downloaded as RBZ files. ๐ฒ
Download it and save the RBZ somewhere logical - like your Downloads folder.
Within SketchUp open the 'Extension Manager'.
In its dialog use the 'Install Extension' re-button.
Find the RBZ and it'll be auto-installed into the correct folders etc.
When it finishes use the tool as outlined in its usage guidance notes at the start of this post, or in the PluginStore's 'More Info' link...
Quick-tip - Tools > ShadowProjector > submenu items...