You can do the following:
WAP(config)#ip http authentication local
WAP(config)#username <username> password <password>
This will set the web interface to use the local userdatabase for login, and add a user.
As mentioned you can use the "show run | inc user" to check users configured already.
That said, it should have worked with blank user and the enable password with your current config. I'd do a show run | inc enable just to see what's configured. Might be an issue with "enable secret" and "enable password". I usually don't use the web interface, so haven't had this problem myself.