I'm having issues uninstalling DTB as well. I messed up during install, and unchecked "Modify File Permissions" because I was installing on a computer where I was administrator. However, I didn't realize it needed to remain checked if I wanted to use an extender, and so it failed to load the add-in on my Xbox 360.
I've also tried to follow the manual uninstall guide, but I was only able to remove the Registry Keys. I was unable to execute the command prompts. I think something is fishy with those double quotes in your guide. Also ran into Ron's issue with the Microsoft site.
So, where do I go from here? I'd really like to get this issue resolved, so I can get commercial skip up and running finally!