Sorry to say you can't just drill a simple hole. You need a milling station to create an under cut (T shaped) slot for the kensington lock to fit and grab.
For now your best option is to write a strongly worded note to Apple. Hopefully, they see this is a major issue for people and rethink this.
If you must do this your self you'll need to find a good machinist to see what he can do to mill the case for one.
Here's the just released iFixIt guide MacBook Air 13" mid '12