You need to configure MC to use your external tool
Right click on 'a' track of that Album in Playing Now->Send To->Send To (external)->Add /Edit Programs
Then click Add, in the dialog that pops up
Name : Album Art
Program Path : C:\Program Files\AlbumArtDownloader\AlbumArt.exe
Parameters : /artist "[Artist]" /album "[Album]" /path "[Filename (path)]"
OK
[Album], [Artist], ie anything enclosed within square brackets corresponds to a field in MC, you can pass any field you like, i've double quoted them to handle tags that might have spaces between words
then select a file->right click->Send To->Send To (external)->Album Art
Album Art should appear in the top of the drop down menu with future right clicks. You will have to take care to only select one file from the album to invoke the tool otherwise the tool will be called as many times for as many files selected.
i'm not sure how well those tool options will be passed from MC, also the path option requires there be a folder.jpg at the end of the path, which is not possible using the method described above in which case a batch file or vb script might do the trick. You would pass the tags to the script which woudl then call your tool.