OK - let me ask a couple of basic questions then.
1. do you have a physical AUDIO cable connected between the AUDIO output of your Korg to the AUDIO input of the Presonus? This is the only way to record the audio output onto an audio channel in Logic.
2. If you are using inputs 1 and 2 for your AUDIO input, do you have the input knobs on the front of the presonus turned up?
3. Have you added an audio track to Logic, and assigned its INPUT to the appropriate channel of the presonus? (input 1-2 if stereo)
4. In your Logic Preferences, under Audio, have you set your inputs to be the Presonus inputs?
5. Have you enabled recording on the new audio channel? The red "R" button should be flashing, and you should see level on the track meters when you play your instrument.
6. If you already have the instrument recorded on the midi tracks, you should then be able to hit the RECORD button on the transport, and watch as the MIDI tracks control your keyboard, and the AUDIO from the audio out of the keyboard is recorded to the new enabled audio tracks in Logic. (Note, you should also make sure that the MIDI tracks you already recorded are NOT in record mode for this pass)
If that isn't working, then double check your physical audio connections. Finally, in order to hear just the recorded audio tracks, you will need to disable the two MIDI tracks you previously recorded, presumably by muting them. If the output of the newly recorded audio track is sent to the Presonus, you should hear the recorded audio track playing back. And of course, you should see analog audio waveform on the recorded track.
If that doesn't work - then I think you may need to contact presonus tech support. But first, I suggest you carefully read through the pdf manual that comes with the presonus (or that you can download from their website) .
One last thing - I noticed in the Presonus manual that the hook-up diagram suggests using the SPDIF connection from a keyboard to the Firebox expansion cable. If this is the way you have connected the unit, you will have to select different inputs in Logic. On the Firebox, those would be inputs 5-6. I have been assuming you are using the ANALOG outputs of the Korg, and the ANALOG inputs of the Firebox above. The SPDIF connections are digital, and should only be connected to another digital input/output set.
Hope that helps.
Dave