With that theme you can choose to display text in the “Appearance-Header” option area. (note: this will close up the header area entirely)
Alternately (what “I” would do) is use your logo as a background image for the site title and simply remove the description code.
In your CSS you could use:
h1#site-title { background:url(imagepath.png) no-repeat 0 0; width:100px; height:75px; text-indent:-9999px; }
h1#site-title a { width:100%; height:100%; display:block; }
Obviously, alter the dimensions and image path to your logo.
Removing the site description code in header.php:
<h2 id="site-description"><?php bloginfo( 'description' ); ?></h2>
Or hide the description with CSS:
h2#site-description { display:none; }
I don’t really work with any themes other than my own but I am pretty sure this is a good solution.