I think I may have found at least a possible cause to my problem when reading through that "Digix Pinout Documentation" thread. In it MichaelMeissner mentions in his Jan 9, 2014 @ 10:45am post:
"In looking at photos of the Due, Mega, Leonardo, and DigiX, it answers a question I've had in looking at various library code. I've seen various library code that does I2C that uses the 2nd I2C port instead of the first i2C port. It looks like yet another case of the Arduino team changing their mind as new boards come up.
In the Uno R2, they used A4/A5 as the I2C pins. Then when the Uno R3 they come out with the new shield layout, and add 2 pins next to AREF for the I2C pins, and wire A4/A5 to use it.
Then the Mega comes out, and it has SDA/SCL relocated to pins 20/21, but I believe it still uses the 'common' I2C pins next to AREF."
If I have time @ work today, I'll look deeper into that MMA_7455 Library to see if THIS MIGHT BE WHY the program is working on my Mega board and not the Digix -> because it may be looking to run off the 2nd I2C ports which are at pins 70 & 71 up by AREF (?).
If I don't have time during work, I will check it out when i get home. I'll also keep looking.
Russ from Coral Springs, Fl.