How can someone make an appeal to un-delete an answer that was deleted by a moderator?
There should at least be an appeal/vote process, if the deletion was unfair. Moderators are humans like rest of us, and they can be unjust.
Flag your answer as 'other' and type 'I would like a different moderator to review this deletion'.
If the reviewing mod agrees with the action the first moderator took, we'll decline your flag with a short explanation that you'll be able to read on your flag summary page.
We can't possibly entertain appeals for the thousands of answers that are deleted every day, meta would just explode. The flag system is definitely the best starting point.
If after that, you're still not satisfied, bring up the topic in the per-site meta. (but be much more specific, e.g. actually link to the answer).