You will need to change the OAT values of your BASE.
Depending on whether you are talking about +15 deg (incline) or -15 deg (decline) relative to the Z axis. Probably something like any of the below for OAT (assuming OAT are already at null values):
Incline: 90,15,-90 or -90,15,90
Decline: 90,-15,-90 or -90,-15,90
Note:
When just creating an incline/decline at the current physical location, you would usually ensure O and T values have the same offset from their current values and in opposite directions, and then A value will be 'your angle of dangle' so to speak.