Are there any messages in aMule's log that seem relevant? Also, check console.log using /Applications/Utilities/Console.app.
Have you tried quitting aMule and restarting it? It may kickstart the process.
What is the file's status? "Waiting"?
If nothing helps, you can copy the XYZ.part file from the Temp directory, give it a sensible name, and just use it. Then cancel the download. The problem here is that aMule didn't get a chance to verify the file's hash, so it may have corruption. Unlikely, but possible.