1) Follow Joshua’s answer: “Help->Install New Software… Select: Galileo - http://download.eclipse.org/releases/galileo Under Programming Languages choose: Eclipse Java Development Tools 3.5.x…”
2) Then go to Eclipse -> Preferences -> Java -> Installed JREs.
3) Click ADD.
4) Under ‘Installed JRE Types" you should see the previously missing “MacOS X VM”. Select this and click NEXT.
5) Click the BROWSE button next to the JRE home field, and the dialog box should say “Select the root directory of the JRE installation”
6) Browse to /System/Library/Frameworks/JavaVM.framework/Versions/1.5.0/Home
7) Click OPEN.
8) In the JRE name field type “JVM 1.5”
9) From there you should just be able to click FINISH and have your JRE available.
I used this to fix a missing JRE error building a Google App Engine project.