This indicates that there is no Java SE 6 runtime available on your laptop.
Normally, however, when there is no Java SE 6 runtime available and you try to launch yEd by double-clicking yEd.app in the Finder, this will trigger the "Download Java" dialog. If you confirm the dialog, OS X will download and install the latest Java SE 6 runtime (from Apple).
After that, double-clicking yEd.app in the Finder will run yEd as usual.