There seems to be an issue with the steam button template, where the form elements on the right side won't work if the button is included into the top navigation in the header template.
Solution is to edit the stc_signinbutton template, change <span to <div, </span> to </div> and add this to the CSS > additional.css template:
Code:
div.stc_signinbutton {
position:relative;
left:16px;
top:2px;
width:auto;
float:left;
}
ofc you may edit the position details according to your template.
I'll add another updated version tomorrow, for now the template/css fix should work.