The LDS fluid is in the suspension system. The reservoir is under the bonnet and over the driver's side front wheel. It is a greyish plastic container with a screw top. Don't confuse it with the coolant tank. It may have a small label attached "Total LDS" To check the level, start the engine, put the suspension to the lowest setting, allow to stabilize then switch off. Unscrew the cap and there will be a slight hiss. You should be able to see the fluid at the bottom of the neck. If you can't, it needs topping up with Total LDS. (Use nothing else. Any other fluid will harm the system. When the level is correct, replace the cap, start the engine and move the height lever to normal.
There are several other reasons for the stop light being on. It's really a matter of finding it. The quickest way would be looking at any fault codes with a diagnostic meter.