A Mechanic reports an APU inlet diverter was damaged during their installation of an APU into an A320. He and another Mechanic failed to see the ladder on the lift platform contact the inlet diverter.

Narrative

Myself and another AMT were assigned the job of replacing the APU on an A320 for cause; using AMM 49-11-98. The APU was removed using the above AMM. The new APU was prepared and placed on a Cargo King to lift the APU into it's compartment. A ladder was also placed on the Cargo King to gain the necessary height needed to install the APU mount nuts.I was positioned on the left; aft side of the APU compartment to observe the left side and exhaust duct for clearance. I was also the Mechanic operating the Cargo King lift controls. The other AMT was standing on the ladder and positioned on right forward side of the APU compartment. His upper torso was above the APU; so that he could observe the right and forward side of the APU and at the same time align the APU mounts and install the mounting nuts.We needed another inch of lift to be able to install the nuts. The AMT on the ladder said 'up.' Neither of us noted any clearance issues. I noticed that the Cargo King would no longer raise. No unusual sound was heard. It was at that time we noted the damage to the APU inlet diverter from the ladder making contact. We lowered the Cargo King a few inches; enough to reposition the ladder. The Cargo King was then again raised to finish installing the APU mount nuts. At that time we notified our Supervisor of the incident.