Not trying to attack you but here's basically what the ACC wire does.
When the ACC wire is "hot", then the device is on. It's basically a switch that comes from your key, that tells your radio/amp/anything else that the engine is running.
So, your amp has three power input's. Well, technically only two, but we won't go into that. Ground, Battery, Accessory. The ground is the return, and should be obvious. The Battery is hooked up to the battery. With a amp it is a good idea to actually run it to the battery. I also run the ground to the battery in the same fashion. Otherwise you can get Ground Loop's. The Accessory comes from your radio. Your radio turns this on when it's on. NOT related to the accessory that your key uses.
So, the amp should have the acc wire wired to the "amp" wire on your radio. If the radio doesn't have a amp wire, it should be on the power antena. Gerenally it's a blue wire. The amp wire is a blue wire with a stripe of white, generally.
Since you are hooking up your computer directly to the car, without a radio, you have two options.
1) Hook up the amp to the accessory wire coming from your wiring harness for your stereo. The problem with this is even when you have the Computer off, the amp will be on. And if the key is in the acc position, the amp will be on. This could run the battery dead.
2) Make your own relay that turns on when the computer is on. And hook your amp to this wire.
If I was ditching the radio, i'd use the relay method, but I can't live without my Sirius.