I'm sorry, I obviously misread your post.
Are you looking for frequency response compensation (FRC) or sensitivity calibration? If you are looking for FRC, it is not supported in SoundMeter. FRC only works with FFT-based spectrum analysis.
If you are looking for sensitivity calibration, then you would need to enter the iMM-6's acoustic pressure sensitivity in the Input Channels calibration screen (there is calibration tutorial information in SoundMeter's built-in documentation).
The iMM-6's calibration file includes the mic's overall sensitivity on the first line. (You can download the file directly at http://www.daytonaudio.com/iMM6_Lookup_Tool/iMM_Text/99-00192.txt
, where you replace "00-00192" with your own mic's serial number.)
The nominal sensitivity for the iMM-6, given by Dayton, is -40 dB, where 0 dB = 1V/Pa. To convert that to V/Pa (to enter into SoundMeter), you need to raise 10 to the power of the sensitivity divided by 20. That is, 10^(-40/20) = 0.01 or 10 mV/Pa (which Dayton also gives as a nominal sensitivity). The sample file they provide has a sensitivity of -37.4 dB, which equates to a sensitivity of 10^(-37.4/20) = 13.49 mV/Pa, which you could enter into SoundMeter either as "13.49m" or "0.01349" (SoundMeter can recognize the "m" and format the number appropriately).
Keep in mind that you need to have the headset input device units set to "V" and the headset input channel units set to "Pa" in order for this to work. Also, you'll still have a nominal calibration. If you need a more accurate calibration, I would recommend performing a relative calibration against another sound level meter that you know has been accurately and properly calibrated. (I recommend this, regardless of which or whose iOS sound level meter app you are working with.)
Please let me know if you have questions about any of this.