Follow normal installation procedures to install IJFR on a Mac computer. However, because IJFR is designed to hook into the OS as a OS X Service, you will not ever run the IJFR.app application.
IJFR is not run like an application. It is always available to you with a simple right-click on a selection of RAW file(s) or a folder of RAW files. When the context menu pops up just selected Instant JEPG from RAW from the Context | more... menu and choose the settings you wish from the displayed dialog. Then click on Extract. It is that simple and fast.