faceplant,
The Edison Arduino IDE I2C defaults to I2C-6; to invoke I2C-1 also a small change is required in the appropriate library function. Maybe you figured this out.
On the MPU9x50 sensors, you have to place them in bypass mode by writing an 0x22 to register 0x37, IIRC. You can also place the MPU9X50 in master mode but it takes a bunch more register configuration writes to do this. You probably don't see the MS5611 because the MPU9150 device by default is not in bypass mode and other devices slaved to the MPU9150 can't be detected until this is changed by the user.