You might use the Firefox Network tool instead (Tools > Web Developer > Network), where the Headers will be displayed (select one of the GET line, e.g. the first)
Another way is to use the Firefox Console (Tools > Web Developer > Web Console)
Let's say your password is "1234abc"; then write and execute this in the console:
btoa("admin:1234abc")
That should give you the <AuthCode> (the example above gives "YWRtaW46MTIzNGFiYw==" which is just an encoded form of "admin:1234abc")
As for "The request is forbidden" message, this is from DLink developers that fudged the web UI (which is why belnox needed to include the Referer string in his example above, a spoofing that nullifies any pretension of security from DLink).
You can go to the home page of your camera, (http://YOUR_CAMERA_IP) from which you can access the other pages. The home page is sufficient to extract the AuthCode from the Headers (if you're not comfortable with the Console trick above).