Print cartridge will not install on HP LaserJet...

Question

After replacing the black cartridge on my HP LaseJet 3800n, I get a message on the printer display 'black cartridge not installed'. I have taken it out and back in and it did not help.  I don't know if it means any thing to the problem but I notice a small black plastic piece that looks like it might be out of place but i don't know if that is so or where to put it without causing possible harm!  Any suggestions?