If of 60ma is the max (continuous) current through the diode. You probably want your opto-coupler to work a little below that. Let's say 10ma.
That means a series resistor of about 90v/10ma = 10k.
You will then find the Ic corresponding to 10ma of If. Figure 6 shows a 1:1 current transfer ratio (but at 10v Vce). So assume that's true for 5v Vce as well. Your Ic is 10ma. You want this kind of current to generate a logic '0' on your avr's input. Check the AVR's datasheet and I think anything less than 1v is a logic low (for a ST pin). That means Rc = 4v / 10ma = 400ohm. Pick 330 or 390ohm.
You will then need to calculate at what threshold input voltage that your opto-coupler will product a high -> make sure that it is over the AVR's input threshold for logic '1'.
As you can see, there is a lot of assumptions / guesswork here so you will experiment a little bit.
What you will find is that using a zener will greatly help producing a sharper turn-on.