Hmm, strange that it worked for me. I can double-check things, but it's mostly identical to the older OS X builds I used to make. There are two places we can look:
1. At the most basic level, can you fsck the .dmg file and get any errors? If not, when you copy the .app out of the .dmg file, can you navigate into it as you can any other folder (e.g. "cd ~/Desktop/RText.app")?
2. Can you check the Console
app (and or "syslog -c" perhaps from a terminal)? Maybe one of these places will record the exact error (bad info in plist or whatever).
Another thing to try: does the ancient 1.4.0 Mac release
(this was the last release with an OS X binary before 2.0.5) or the 2.0.5
I'll grab the binary off of SourceForge and do the same tests.