Ah, this method for creating custom items (i.e., for the files you have attached) was phased out after v2004. For the most current method which is MUCH more flexible, see the current user's guide regarding the creation of custom items, and you might also want to check out:

http://www.empirisoft.com/support/showthread.php?t=1315

Check out the attachment that is located there called slider_revised.zip. It created using the newer method to create custom items in MediaLab. It's done with javascript so it can be modified pretty easily to do whatever you want. You could simply add an embedded image, video, or flash file (etc.) to the custom item's main html page.

Note, if you still have v2004, there isn't really a convenient way to get around this without modifying the exe file for the slider.