The problem is most likely that you downloaded just the Eclipse SDK, not the full version of Eclipse, including the PDE, from eclipse.org. Some of the sites distributing Eclipse don’t package it exactly the way eclipse.org does so that it’s not recognized. When you list the eclipse directory from a terminal window with ‘ls -al’ there should be a file called .eclipseproduct. Without the proper packaging, the installer can’t verify the version properly so it will refuse to install, rather than risk installing against an untested version of Eclipse.
Please download the full 3.0M4 build for Mac OS/X from eclipse.org and be absolutely sure to follow all their installation steps to the letter (ie. Safari browser, StuffIt version, etc). Once you’ve done that, MyEclipse should install just fine on it.
–Scott
MyEclipse Support