Playback Issues!!!

I’m running my guitar through a Line6 POD HD300 petal board into cubase on my Mac. I have my amplifier hooked up to my petal board also, and this is why. I have no external speakers hooked up to my desktop- however, the speakers on either side of the computer do work, as the mac itself is only a few months old. BUT: I cannot get any form of playback through my computer after recording. The computer seems to be using my amplifier as a stereo output, in that anything I record on cubase will only playback through my amplifier, rather than the computer. I can still record when the amplifier is unplugged, I just can’t hear anything until I plug it back in. This is very aggravating, as it is impossible to pan the tracks into the left or right side of the speakers, since the only speaker is my one amplifier, which, no matter how I pan the tracks, always plays them as if I had centered them. Using headphones has no effect on being able to hear sounds coming from the computer. How do I make the program use my computer speakers as the sound output (cubase 6.0.5)?