Hi Kai,
per your recommendation I installed MacPorts on OS X 10.6.8 and then used it to install Meld successfully. But how do you open Meld? Typing "meld" in Terminal only returns warnings and X11 opens without anything to show.
I read your instructions in /meld-1.8.4/INSTALL but I don't know if these apply to Meld when installed by MacPorts. Could you please provide instructions in an easy-to-read fashion similar to these at http://kdenlive.org/download-mac?