[JohnM]

Apple provide only limited support for the JavaSound classes, preferring that people use their (platform-dependent) CoreAudio classes instead. For REW, which uses JavaSound for cross-platform portability, that means only the on-board audio input is supported, though off-board audio outputs may be accessible by selecting them as the default in the Mac's audio preferences. Leave REW set to use the default input and output, but set the sample rate to 44.1k, it may then work. There are various display bugs on the Mac also, which have been fixed in the current beta build but that is not yet available for download.