I had the same problem--had never created a 'mysql' account and was mystified when the computer told me the account already existed. But somehow, probably during installation, the account is created by the computer behind the scenes. To check this, go to NetInfo Manager (in Utilities), and click on the "Users" field. You should see mysql there, along with some others you didn't know you had, like "nobody", "root", and "www".
So it doesn't actually need to be created. You might have to do something with it in NetInfo Manager, but I can't remember.