Mostly likely the problem is that the installer is trying to run itself with a VM that it found on your computer that is NOT a Sun or IBM VM (Probably gcj). You can either a) trouble shoot this problem or try and force a JVM to us by way of the LAX_VM environment variable, OR you can download the manual install and use that. The manual install is really a 2-step process that is detailed with a nice step-by-step installation guide that outlines everything for you. All you really need to do is: 1) unzip MyEclipse, 2) fire up Eclipse and add MyEclipse as an extension via the Help > Software Updates > Manage Configurations menu.
Of course the manual install goes into more detail than that, but this is probably the easiest way.