sorry bud, but i am on the latest build also, and when i go to mail, settings, accounts, add account, then scroll to the bottom of the pop up window, i get exactly what HG is getting. so i'm not sure what your system is giving you, but something must be wrong on your system, as i'm getting the exact same thing he is, where you can add a user name.
it's there, man. if you look carefully at HG's post, it is there. go to mail, click the settings icon in bottom left corner, click on accounts, then at the bottom of that list, is says add accounts, click that. a window will pop up in the middle of the screen, put your cursor in that window and scroll down to advanced setup, click it, and then click internet email. in the next box, you can pick account name and user name, among other choices. check it out, man. it's all there. these new apps just need a little getting use to, that's all.