I am using Mathematica for students 18.104.22.168 (WinXP Pro).
to load the package and then
ShowGraph[ CompleteGraph ]
but there is no output showing the graph.
I have also tried it with another graph defined by myself and it does not work.
When I type
I can see that the DiscreteMath`Combinatorica` package has been loaded.
If I compare my commands with the help text for the package, I think it should work like that.