Setting up a BOGO deal can be done the following way.
Select the offer type of "Buy X for $" and enter the offer details as shown below.
Next we'll need to make the messaging appropriate customize the product / upsell messages, instead of the standard discount pricing message. This will reflect the BOGO intent.
The system will not automatically add the second item to the cart. That must be done manually by the user.
To do buy 2 get one free just use 33.333% as the discount instead of 50% and set the quantity to 3 instead of 2.