Take a 2 conductor line cord and plug it into the page port on the CPU, connect the leads to the AUX IN connection on the amp.
If you need a momentary closure during page (MOH mute, etc), use a 4 conductor cord and use the outer conductors for the make/break circuit.
To use: dial intercom 70 to external page only, dial intercom *70 for external page plus calling group 1 members.
If none of this makes sense, hire a sound tech for an hour! I can't tell you the number of blown amps I've encountered because the "tech" had no basic electronics skills. The tech hour is far cheaper than an amp and/or CPU.