Adding Administration menus

I’ve spent the past few days trying to figure out adding menus to the admin section for a theme.

I can’t get to add any info without getting a white screen.

I’m using the example on the Codex page:
http://codex.wordpress.org/Adding_Administration_Menus

How do I add code to this section:

// mt_toplevel_page() displays the page content for the custom Test Toplevel menu
function mt_toplevel_page() {
    echo "<h2>Test Subpage</h2>";
}

I try to add text here:

// mt_toplevel_page() displays the page content for the custom Test Toplevel menu
function mt_toplevel_page() {
    echo "<h2>Test Subpage</h2>";

ADDING CODE HERE

}

Whatever I add anything, it is just a white screen.