Thank you for your feedback. When you select the two rigs for retargeting, the first is the source rig and the second is the target rig (active armature).
When you asked where to add the retargeting system, were you referring to a preset? I'm currently preparing a video about presets, stay tuned.
Whoever made this did a very good job. It’s such a big and impressive feature, but there isn’t any detailed video about it. It’s hard to understand where to select the source rig and where to add the retarget rig. There should be at least one detailed tutorial video explaining this properly.