A while back (late 2007 to be exact) Anders Holck posted a Mac helper app that lets you double click an R3D file to open it in REDAlert. The link was only ever posted on reduser and disappeared at some point after it was posted.
Happily, it has recently been re-posted as a .dmg installer file. Once you’ve installed it, select an R3D file in the Finder and in the Get Info box (cmd + I) set RedPortal as the app to open all R3D files (Open with RedPortal and then Change All).