This should work
<div>[mura]$.dspInclude('themes/rescue/display_objects/custom/main.cfm')[/mura]</div>
Your include is being called from the includes folder, so you need to include the themes folder and the rescue folder to get to the right path.
For consistency across themes, you might want to use Mura's setting for the theme name, but I would have to look that up. If you are ok hard coding it, this should work just fine.