How can I enable automatic resume on lid open?

This needs to be supported by hardware through a sensor for the lid-open event. Unless your hardware supports it, there would be "no-one" who will get notified of the "lid-open" event, and wake your laptop.

Have you seen this post? It looks like the method worked for one guy but not the other, so its not a guaranteed fix, but its worth a shot. : )

Here's a brief summary:

You can check whether the Lid open event is configured to wake up your OS by executing
cat /proc/acpi/wakeup. If the status of LID or LID0 is disabled you can try enabling it by executing

sudo -s
echo "LID" > /proc/acpi/wakeup

This will set the status to enabled, and executing the commands again will set it back to disabled.