The RealZip plugin was provided by a third-party developer whose website has been offline for several years. You can still grab the plugin from the Internet Archive's copy of their site.
Extract the .rbx file into the plugins
sub-directory of the REALstudio install directory and restart the IDE. Since this plugin is no longer maintained, there may be issues with using it in a modern version of the IDE (the included demo project works, though, with only a deprecation warning unrelated to the plugin.)
There's also a GZip plugin available on SourceForge (similarly unmaintained) and the commercial MonkeyBread Compression plugin which is actively maintained.