Here is a link to arduino mega code, for APM2.8, that reads the MS5611 temperature and pressure, via SPI.
Link: GitHub - rwsenser/APMhelpers: APM 2.8 Arduino code to access APM resouces like the barometer, etc. then download via zip and take ReadAPMbaroMS5611.ino
This is a standalone sketch that shows the steps needed to use SPI with the MS5611 in an APM 2.8. It has been tested on a single APM -- but the values it shows are correct for central Colorado, USA.
Perhaps this will be useful as there do not seem to be many (any?) working MS5611 libraries that utilize SPI.
Thx,
RW Senser