Mark,
I just want to point out that JadEclipse is not supported officially with MyEclipse.
I noticed there is no version of JacEclipse that is compatible with Eclipse 3.4. Since you are working with MyEclipse 7.5 you need a version of the plugin compatible with Eclipse 3.4 because that is the base eclipse version of MyEclipse 7.5.
You can downgrade to MyEclipse 6.x and try the plugin compatible with Eclipse 3.3.