Adding Directories To Path Failure?

How do you actually add directories to path? Every tutorial I've come across just says tack the path on to the end of the PATH variable after the previous variable's semicolon. That doesn't work. For instance if I want to add a path B:MinGWin I would just add it to the system variable PATH like soand now anything in the bin directory of MinGW should be in the path. Why doesn't this work? It still pulling the "this is not an internal or external command" nonsense on me. I've also tried adding directories on the C drive to path and ended with the same results.