Halo 3

Just go to the Giant Pit that is in the middle of the map and go to the higher of the 2 roadblocks. Go into forge mode and look straight ahead. You should see a giant pane that has one bolded line and some little lines to the left and right of it. Look to the right and up and go all the way up to the top and you will find it. The video is easier to watch and listen to and it explains how to grab it once you found it.

Second

Use this video:
(This link will take you to the exact point in the video where it shows this skull)

Otherwise, you can go to 0:21 using this link:

For those of us who aren't fully versed on all the Halo lore, etc... I'll say this plainly so that you don't have to go hunting for the answer like I did.

In order to get all 6 of the Vidmaster Challenge: Brainpan skulls, *all you need* is Halo: ODST. I'm saying this so that you know you don't have to buy the Mythic pack for Halo 3. ODST has those maps on the second disc, and those maps do contain the 3 skulls from Halo 3.

So in other words, if you have ODST, you can get all 6 skulls, including this one, on the multiplayer disc (very, very easily, and get yourself a quick 175 GS in under an hour), or you can use Halo with the two Mythic packs. All of the skulls are available in both places.

I highly recommend the videos that ryankelty posted in the Vidmaster: Brainpan guide. They are excellent, and I want to give him credit for finding them (and to the players who posted them), although I'm going to put them up here for easy reference.

This skull, Orbital, is in the first video. The second is for your convenience if you're looking for all six.

Cheers!

Use this video, I used it and it saved me a whole lot of time versus me finding it myself:

http://vimeo.com/3458939

Thanks to Adam Pisani for this video.