Do you see an icon called "viDrop" on your desktop?
Where do you see the file "mplayer"?
It's strange because when I install viDrop on my computer, I don't see a "mplayer" file, I just double click on the viDrop icon on the desktop and it opens up the program.
You did download viDrop from here:
viDrop 0.57 AVI XVid Converter?