John - firstly, these are usually infra-red, so it may well have been illuminated even if you couldn't see it (hint - you can often get a mobile phone camera live-view to witness IR LEDs).

Reverse voltage?, yup, a good way to fry an LED.

The good thing is these are generally fairly standard devices, although pin-outs may vary. And drive voltages?, can be pretty much whatever you need. I'll assume you've not hit this before otherwise you'd not be asking.. Take the supply voltage, and subtract the forward voltage drop of the LED. Then divide that by your chosen forward current. That gives the value of a resistor to place in series with the LED for it to operate at that voltage. That Datasheet indicated a max forward current of 40mA - that's an absolute max rating, a rough rule of thumb is around 10mA.

So, if you're trying to drive at 5V, and the Forward Voltage is 1.8V, with a Forward current of 10mA (0.01A), that's (5.0-1.8)/0.01 = 320 Ohms, nearest easy found value is 330 Ohms. If you want to drive at 12V, then (12.0-1.8)/0.01 = 1020 Ohms, nearest easy value = 1000 Ohms (1k).

Whichever resister value you calculate, you place the resistor in series with the LED then drive that from the DC supply.

Note, its generally considered unhealthy to reverse bias an LED - they go pop at quite a low reverse voltage. Even if they don't pop, then can be significantly degraded.

Amazon sell reflective opto-couplers - there's generally not a huge difference in the 4-pin devices (2 pins for the LED, 2 pins generally for a photo transistor). They should be pretty much interchangeable.