Hello and welcome to Justanswer!
When the tank is that low adding 1.5 liters probably isn't going to be enough to turn the warning off. There are two different sized tank options depending on vehicle build, one holds 12.8 liters and the larger one holds 19.2 liters. I'd recommend filling the tank completely.
The bottles of DEF 1/2 gallon each from mercedes screw on to the DEF filler entrance but I cannot figure out how to get the fluid to flow. I cut the first one off and poured it in with a funnel. What is the proper way to use these bottles--i.e. how do I get the fluid to flow in?
How do I know which capacity I have--12.8 or 19.2 liters?
A dealer would have to look up sales codes by VIN which system you had to know the capacity.
After the cap is removed from the bottle you put the bottle onto the filler neck, gently push down and turn the bottle clockwise until it stops, and then you can push down on the bottom and the DEF will flow.