The OS X version of REW (V5.0) does not know about UMIK-1, SPL readings for OS X need to be calibrated using the procedure in the help (Calibrating the SPL Reading).1st Problem:
On both Mac (REW v5) and Windows (latest REW beta), the SPL readings appear to be way off. In my quiet apartment, the UMIK reports about 65-75 dBSPL, C-weighted. I have a standalone SPL meter which reads about 10-15 dB below this. I would tend to trust the calibration on the UMIK over my cheap SPL meter, but I think the UMIK reading is way to high for just the ambient noise level. These measurements were taken with the UMIK cal file loaded in to REW in both Mac and Windows.
For the Windows version (assuming V5.01 beta 11 or later) did REW automatically detect the UMIK and ask if you wanted to use it? If not, please set the input device back to default on the REW soundcard preferences then shut down and restart REW, it should ask if you want to use the UMIK-1 and then ask for the cal file. If you had already done that, please post a screenshot of the REW soundcard preferences dialog.
Is the UMIK selected as the default device and REW set to use the default input?2nd Problem:
When using the UMIK and REW on Mac, REW reports that the UMIK is clipping when taking measurements. This happens even when using my Macbook speakers for output, so I'm sure the sound pressure is not actually exceeding the range of the UMIK. I tried adjusting the UMIK level in the Mac OS MIDI Setup, but it did not appear to have any effect on the results. I am using a 128k sample length for these measurements.