Apple calls UPnP....Port Mapping
Open AirPort Utility - Click Manual Setup
Click the Advanced (gear) icon
Click Port Mapping (If Port Mapping does not appear, you have the Time Capsule set up in Bridge Mode. You will need to set up UPnP or Port Mapping on your "main" gateway or router in this case)
Click the + (plus) button at the bottom of the list to set up your service and assign specific port numbers according to your webcam documentation
You'll probably have a lot better luck on this if you post in a D-Link Forum. It's possible.....but very unlikely that a user that is familiar with advanced settings on a D-link router will be checking an Apple forum.
The dcoumentation on your camera should provide some information about what ports and services will be required.