Brigadier, the normal value for the Temp directory preference setting is: /Users/yourusername/.aMule/Temp
For example, my username is "ken" so my Temp directory is: /Users/ken/.aMule/Temp
stefanero was almost right, but Mac OS X uses "/Users", not "/home".
Once you reset that value, you should quit and restart aMule. Then, all of your downloads should reappear.