If you have the board you linked to, you have a Pro Mini 3.3V/8MHz, which makes a difference as the default is the 5V/16MHz version.
Have you tried to change the interrupt trigger signal as I suggested? This probably will already fix your issue.
I don't know how the ADXL345 reacts on a floating bus, it might simply run as expected but it might also decide that no one is listening and don't activate the interrupt.