If there is a rectangular box between the handle and the pipe, that box is for the valve guillotine to slide into. So to open the sewer you pull the handle allowing the guillotine to slide into the box and thus open the pipe to flow.
BTW, the instruction to open the black valve first is so that you open the gray water valve second, thus flushing out refuse from the black tank if any remains in the piping. DON'T OPEN THE BLACK VALVE BEFORE REMOVING THE DRAIN CAP OR HOOKING UP TO THE SEWER DRAIN! (My apologies if this is too basic of a warning - but you never know.)
Since the black handle has been pulled out already, push it back in, but put a bucket under the drain cap before removing.
