You can access the login page directly with the URL
http://www.[yourblogsite.com]/login.aspx. IOW, just add /login.aspx to your web address.
If you mean that you want to have the link to the login page truly open in a new window, you could add a "target" attribute to the link that is at the bottom of the theme,
you would look for the tag like this: <a href="/login.aspx"
id="ctl00_aLogin">Log in</a> (it's at the bottom of that template you linked to) and change the html to this:
<a href="/login.aspx" target="_blank" id="ctl00_aLogin">Log in</a>
It's that target="_blank" that sends it to a new window.
On the other hand... if you're having some completely different problem logging on, please explain further.