Only lowercase letters (a-z) and numbers are

I am trying to add the words (User Name) to the error message when people are signing up to my site. For some reason if they miss inputting data into a field they get a pop out window telling them but if they put a user name of say something like AA_AA then they get a message at the top of the sing in form Only lowercase letters (a-z) and numbers are allowed. But the problem is it doesn’t say User Name so people are not sure what they have filled in wrong.

Can anyone help? Also is there a way to redirect people to another page after they have submitted there user form because again when they click submit form they just get a message saying a email has been sent to them but it is at the top of the page and some people don’t see it. It really needs to be a pop out window.

Can you please post a screenshot for us? Thanks!