It operates with OSC, and so you don't have to go into MIDI-mapping mode to assign a slider. Just click the parameter in Live you want to control, then move an unassigned slider. The name of the track/device/parameter shows up on the TouchOSC display around the slider. You can un-map directly from the iPad.
The mapping is saved with your Live Set, so you can switch between projects and the sliders/labels will update automatically. It uses zeroconf auto-discovery, so Live on the computer and TouchOSC on the iPad can find each other easily.
https://github.com/zsteinkamp/m4l-zs-Knobbler3