Hi Rachel,
The answer your first question is yes, you should be able to simply connect the line out of the KN1400 to the line in on the Mac.
If you have a suitable external amplifier, try connecting the the line out of the KN1400 directly to the line in (high level) on the amp to check the that you are getting signal from the KN line out.
As far as the Mac/PC interface is concerned I have used it only in the PC mode where you use a cable to connect directly from this Mini DIN socket on the KN to the RS232 (Comm) socket on the PC when a MIDI socket is not provided on the PC (e.g laptops). A suitable software driver was provided to facilitate this. I presume the Mac application is somewhat similar.
Cheers
Len C.