How do I align these buttons in one line?

You wrapt the sign in / up button in a div, add a float: left to that. Then you need a clearfix after that, you can use the helper where you have the css for welcomemessage:
{xen:helper clearfix, '.welcomemessage'}

Then you need to position the buttons after this, I think if you add a div container to both buttons, it should be easier.
Top Bottom