It took a few minutes to track down, but here’s a quick note on how to enable Tap-Drag in OSX Mountain Lion. To use: tap once then tap again and drag without pulling your finger off the trackpad.
Go to System Preferences > Accessibility > select “Mouse & Trackpad” > “Trackpad Options” click the checkbox for “Enable dragging”.
“With Drag Lock” means that if you need to take your finger off the trackpad, your next touch will allow you to continue dragging if it is a drag move. To end Drag Lock, simply tap the trackpad.
“Without Drag Lock”, when you remove your finger from the trackpad the dragging will cease.