From your Administrator panel Choose Extensions/Manage and just click this and it will give you the install window.
Drag the mod_tplshowcase_j3.zip file and drop it. Once it is installed you will get a Success window.
For Joomla! 2.5 it is just a little different.
You will want to choose Extensions/Extension Manager.
You will browse to the file mod_tplshowcase_j25.zip and Choose upload and install.
When it is finished you will get a Success Screen.