This script will add a Button to the header.  A typical place to place it would be in "Custom HTML"

<script>
AJS.toInit(function(){
    AJS.$('.aui-header-primary .aui-nav').prepend('<li><a href="http://www.atlassian.com" class=" aui-nav-imagelink">Atlassian</a></li>');
});
</script>