photoresistor in Multisim

You can model it as a perfect current to voltage converter with a diode in series driving a FET -- use the diode in series with the current portion to take into account the required diode drop and non-linearity -- and note that the relationship is far from linear -- it is a diode after all.

somethings else to be aware of -- these optocouplers have "memory" and take milliseconds to respond -- they are sensitive to temperature (like all diodes) and the sensitivity is that of a semiconductor!

Agilent (whatever they call themselves today) has a model for their HCNR200 and 201 optocouplers -- this can be applied to the Vishay IL300 as well -- if you have to do anything requiring precision greater than 8 bits these devices are the way to go.