If you manually put the i2M into update mode (so it flashes red) as you have done, you can simply follow the instructions in the firmware updater manual, which is in the zip file with the updater application.
Where are you double-clicking the .sfu file? You should be able to select the .sfu file, by selecting the "Load file" button in the updater. Once this is done, the updater knows to look for an i2M, and it should then give you the option to update it.
You shouldn't need to double-click the file.
If the .sfu file is on a network drive sometimes you can have problems: the Mac doesn't always treat network files the same as local files. If that's the case, then try copying the .sfu to a local drive.
Are you using the latest firmware updater (version 1.0.2), there were problems in older versions on the Mac.
So, the steps are simply:
(1) disconnect i2M.
(2) Run firmware updater.
(3) Select "Load file" to load the .sfu file.
(4) The updater tells you to connect the i2M.
(5) Connect the i2M while holding down the button, it connects and flashes the logo red.
(6) Press "Update" and it should complete it.