Good work, Chokboyz. I have been trying to get around this problem, and did it in a similar way. For some reason, the LB_DIR message sent to the listbox cannot handle a relative path, but instead only accepts an absolute, so I have to use GetModuleFileName, to obtain the path. Also, you do not need to add a '/' to the path, you can use a regular '\'.
EDIT: I should also point out that, for some strange reason, the SendMessage function returns LB_ERR even if it finds the files and they're in the right place. A bug perhaps?