I've played the main campaign several times, but have never seen the expansion. I've redownloaded and reinstalled the game, but I'm not seeing where the expansion is. How do I access it?

I don't think you can. I read somewhere that the DOS version of the game can't access this content. So even though gog.com upgraded our version to Dungeon Keeper Gold recently, it doesn't appear to offer anything new :(

It looks like the Deeper Dungeons content was just added today:
http://www.gog.com/news/content_levelup_bullfrog_titles

Did you try to download the new version? Since I just bought it (I was waiting for the Deeper Dungeons), I didn't download it yet.

It´s a different exe, just open the folder where you have installed Dungeon Keeper Gold, and execute "Launch Deeper Dungeons".

Yep plus when you install the new gold edition if you have the shortcuts enabled on install you should get two - one for the base game and one for the deeper dungeons

Thanks for the tips. Thank god for GOG.com as well, I just spent 20+ hours doing the deeper dungeons and they were really good!

Alternatively load Deeper.exe in your own copy of DOSBox or under Windows 9x without emulation.