Try now, you had commented out this line in skin/reboot/custom/header/categories_header.tpl
Code:
<label for="menu-dropdown-0" class="toggle_menu_label">{$lng.lbl_reboot_navigation}</label>
Edit, I changed it to...
Code:
{if $main ne "checkout"}
<label for="menu-dropdown-0" class="toggle_menu_label">{$lng.lbl_reboot_navigation}</label>
{/if}