i tried it too, i installed it and it wasnt working well either
almost the same problem, when the cairo-dock appeared with all the icons i tried to eliminate one of them and the icon kept sticked to the screen , and there was a black rectangle that doesn't erase from the wallpaper so i uninstalled it
do u think that mint xfce doesnt have compatibility with this docks???
i was thinking on plank , but it is based on docky so maybe it would bring some problems too. Installing and configuring docky on xubuntu was so easy , i dont know what is this happening on linux mint 13 xfce, this distro is awesome but its very weird this docks issue.
Everything works well , got compiz effects , qjoypad , synapse ,caffeine, netflix, wine , pcsx, all this apps have no problems. I really like this distro but maybe i just use synapse to execute apps or the icons on the desktop to keeping it simple.