Hi,
It is not too complicated to use accelerometer to calculate the angle.
If you use single axis, then the angle is just arcsin[(X/Y axis acceleration in g)/1g]. This could be a easy start for your reference to use accelerometer to calculate the tilt angle.
Refer to interrupt issue, I would suggest you make the activity threshold a little lower, then it is more easier to generate the activity interrupt by rotating or shaking the board. What's more, I would suggst to read DEVID firstly to make sure you can read ADXL345 correctly, then try to configure some registers and read back to make sure you can read/write these registers correctly. If you can do these all right, after you enable data ready interrupt, you should can see the periodic square wave output on INT pin in the frequency of ODR that you configured.
BR
Neil