Hey there,
I am having the exact same error as above. I have created the database and added a user, and am using the cpanelusername_databasestuff for the required fields as advised above. I feel like everything is right, but it still isn't working. So I opened a help desk ticket and received a response. My answer from them was "Your installation is not complete." Uhhh... that's the problem, not the solution!
Is there anything else I don't know? I'm a total newbie at mySQL stuff, so... help? Maybe there's a restriction with the number of letters that your username has to be, or you can't have upper and lowercase numbers, or maybe I was supposed to set something else up that I missed in the instructions besides just a database and a user for that database?
Thanks for any help you can provide!