It sounds like your account may have lost admin privileges for some reason.
Access your database via phpMyAdmin (most hosting providers offer this in their control panel), go to the wp_users table and find the row for your user_login . Note the ID of this row. Now, go to the wp_usermeta table and find the wp_capabilities row for your user_id . Set the value of this row to a:1:{s:13:"administrator";b:1;}
Seems to already be set right – user id 1 (verified this was me) is already set to a:1:{s:13:”administrator”;b:1;}.
The problem is with the plugin. Open mobile-theme-switcher.php. Towards the bottom (should be line 109) find the line
add_theme_page("Mobile theme switch", "Mobile theme switcher", 1, "Mobile theme switcher", "mts_show_admin");
and change it to
add_theme_page("Mobile theme switch", "Mobile theme switcher", 1, "mobile-theme-switcher", "mts_show_admin");
Yep, problem is in 4 param of add_theme_page that can not contain spaces, change it to anything and all will be ok