OK, there are a couple of issues here.
First, the mechanism that wxMac uses to launch the preview program doesn't work when the program has a space in its path or name. Lame, I know. :] So, the trick is to use an app with no spaces in its path. You can move or rename apps to achieve this.
There is an alternative, though. Apple supplies, as part of OS X, the command-line program /usr/bin/open. Its intended purpose is to provide a way to launch programs, URLs, and documents from the command line in Terminal. It can be used to launch a program with spaces in its path and use it to open your .part files for preview. Put something like:
/usr/bin/open -a "/Applications/MplayerOSXB8r4/MPlayer OS X 2.app"
into the preference. Notice the quotes around the application path -- they are necessary if there are spaces.
Even if you have an app with no spaces in its path, you may want to use the /usr/bin/open technique anyway. The difference is that /usr/bin/open brings the app to the front when it launches it. Otherwise, the app will launch behind aMule.
Lastly, the QuickTime Player isn't useful for previews. It uses the file's type code or extension to determine what kind of file it is, rather than looking at the file's contents. Because of that, it doesn't understand aMule's .part files.
I hope that helps.
