Flat vs frequency selective fading in MC-CDMA?
It would help if you mentioned where your references for MC-CDMA and your channel model come from. 

The short answer is that it depends on the level of abstraction you want. For a flat channel, you would multiply by a complex-valued gain. For a frequency-selective channel, I would typically form the time domain signal and convolve with an impulse response (generated from some channel model that reflects your target environment). Perhaps if you provide us with a reference, I can provide additional assistance.