Using onboard audio output in Logic Pro 8

I just got Logic 8 and I have a Mackie Onyx 400F interface hooked up through firewire. I can get input fine from the Onyx, but I can't see any way to route the output to the onboard audio (i want to use headphones on the onboard headphone jack). The program will only let me route the audio to the outputs on my Onyx. So right now I have to hook my headphones up to one of the outputs on the interface, but this only allows me to hear mono out of one side or the other. Is there a way to change output to onboard audio, or will i just have to get speakers to plug into my interface? Any advice is appreciated.