The sender is at the back of the engine below the coolant pipes. In this picture you can see where I've replaced mine with a turbo oil feed line.
It's way back by the power steering pump and such. You'll get to it from underneath the car. At the bottom of the picture you can just make out the axle carrier bracket. The starter is to the left, out of frame.
It's okay to run without it. When it reads low oil pressure it makes the red oil light on the cluster go on. That's it. And if that happens, it's basically too late for the engine anyway, so it's kinda useless. I still have mine hooked up, but that's because I'm sort of OCD about those things.