You could calculate the resistance with Ohm's law: \$R = U / I\$.
However, this does not really make sense, because neither the input nor the output of an optocoupler have a constant resistance:
The input is an infrared LED, i.e., a diode. This means that the voltage between the anode and cathode pins is more or less constant, while the current is whatever amount your circuit sends through it.
The output is a transistor (for most optocoupler models). This means that the voltage between the collector and emitter pins is mostly determined by your circuit (typically, you have a load resistor to drop most of the voltage when current flows, so the voltage is more or less constant), while the maximum current that can flow between them is somewhat proportional to the current through the input LED.
Calculating the resistance does not solve any problem. To help you understanding how it works, measure the current at the input, and how much current can then flow through the output.