NORMA is a Visual Studio extension. Visual Studio runs on Windows, so you'll need a Windows virtual machine to run it on the Mac.
You can view the diagrams from a local .orm file on a Mac browser using http://ormsolutions.com/tools/orm.aspx (one diagram at a time) or http://ormsolutions.com/tools/ormPrint.aspx (all diagrams shown, prints one diagram per page). This is a read-only view that shows the ORM diagrams only (not relational diagrams) and doesn't have a verbalizer or other NORMA features, but at least you can view the diagrams on the Mac. There are also instructions linked from http://ormsolutions.com on how to use these to view .orm files on your web server, so you can share your models.
The viewer is actually a stripped version of an editor that isn't sufficiently complete for public consumption. At some unspecified point in the future you will be able to do .orm editing using this tool, but I can't give you a firm date. Live browser verbalization will come on line before the full editor.
-Matt