Ground Connection When Using AC Wall Wart Supply
Short Answer:

Because the input range of the switching regulator is up to 36V, you can use the (-) terminal as your "ground."

Longer Answer:

The wall-wart (or whatever PSU you use) likely has an internal transformer, which isolates the system from the mains power. As a result, there is no official ground, and the voltage on the secondary side (available to you) is the direct result of whatever AC component is going through the primary. Without a center tap, you don't really have a ±12VDC PSU. Instead, you've got a 24VDC PSU where you can define an arbitrary ground point. You could make a simply voltage divider between the two rails and call that a ground. It would work, but it would be a relatively high impedance (or a high power loss) reference.

Instead, if you insist on a unit without a ground reference, just use the (-) terminal as your ground. The SMPS will work fine, albeit less efficiently, because it's still within the input range. This will mean, however, that you can't get negative voltages from that PSU because you have defined the (-) rail to be your ground.