Most often when you see a plug like that on something as simple as a footswitch only two prongs out of the five are used. The others are there for show. This seems to be an attempt to keep someone from plugging in a fixture from something else, say a footswitch of a competitor. A look inside the unit will tell you if theres any more than that attached. It may even be possible to find a compatible plug at Radio Shack or some other electronics supplier and make an adaptor to whatever you have available.