Can I use mirror lockup for long exposures on the Nikon D80? The option appears to be in the menu, I can't seem to enable it.
The D80 has an Exposure Delay mode, which basically adds a 0.4 seconds delay after opening the mirror, then automatically starts the exposure. So it's not a manual step, but just an automatic mirror lockup. From a quick Google I think it's custom menu open 31.
No you can't. It is only for cleaning the sensor.
If you want to do long exposure shot, you can select the manual mode and roll the rear wheel in 'bulb mode' (after 30").