I have a couple of questions for you so I can help diagnose the problem.
Remember that if you assume that you have checked or have left out any
information relative to the problem, it is unlikely anyone will be able to
help.
Is this a problem only for Excel? If not, the problem likely lies within
your hardware, whether that be the port or mouse is another matter. It is
also possible if you are not your system admin that the right click feature
has been disabled.
Any other information that you may find relavent will help us help you.