After installing wine and steam, I ran steam and I got to the login screen, however you can't type anything as you can't give the box focus.
Having searched around for a while I found a number of forums had this problem with no successful solution. I also found that if you right click in the box, and click copy/cut/paste it gives it focus...however this did not work for me.
I searched around for an on-screen keyboard and found this one: xvkbd
It allows you to set the focus in the box and insert characters. I found that a combination of using the 'focus' button and right clicking and using copy allowed me to type my username and password, and then you can just tick Remember for future logins.