Sorry for not being clear. It is confusing when we have a mat that works on the PC and another one that works on the GC and Dolphin which acts like a GC on a PC...
GC mats that are somehow connected to a PC (via a USB adapter or otherwise) and detectable by Windows, can be used by Dolphin to emulate a GC DDR mat or Active Life (Wii) mat. However Dolphin does not yet emulate DDR mats (which are not the same as Active Life mats) that are connected to a Wii DDR game.
For a list of mat products that can be used with a PC, I would suggest you browse the StepMania website:
http://www.stepmania.com/controllers/
GC mats that are somehow connected to a PC (via a USB adapter or otherwise) and detectable by Windows, can be used by Dolphin to emulate a GC DDR mat or Active Life (Wii) mat. However Dolphin does not yet emulate DDR mats (which are not the same as Active Life mats) that are connected to a Wii DDR game.
For a list of mat products that can be used with a PC, I would suggest you browse the StepMania website:
http://www.stepmania.com/controllers/