If you have a dispute with a diamond moderator, your options are:
Make a post on the relevant per-site meta regarding the action taken, which will allow the community to discuss it.
You can access the site's meta by using the site switcher in the top right corner: click the "meta" link there to open it.
Flag for moderator attention and request a second moderator to have a look.
Visit the site's chat room and ask other users for advice. Note that you need 20 reputation to be allowed to chat, and not be suspended on your parent site. This would also be public, and both the moderator(s) in question and other chat users will be aware of your request.
You can find the button to access the site's chat room the same way to access the meta site: the "chat" button is right next to it.
If all else fails, send a message to the Stack Exchange team via the "Contact" link at the bottom of any page with your complaint and links to the questions/answers. Requests sent there will go straight to Stack Exchange staff and cannot be viewed by site moderators.
For the record, here are levels of moderation (from highest to lowest):
It's often best for you to try to work things out at as low a level as possible. If you have a problem with one moderator, it's faster for you to reach any of the other elected "Diamond Moderators" on your site as they have the most experience with your specific site and may even have seen other actions you have taken that give context to the issue at hand. Sometimes you want to go above someone's head (or even straight to the top) with a complaint or request, but realize that the recipient will wonder why this person with a complaint couldn't resolve it with the team of moderators that were put in place to do just that.