If the mat has been there for quite a while now & she's also had a bath...it's probably really knotted up. I don't think you'll be successfull in getting it out w/out hurting her & you do not want her to associate grooming time ='s ouch time at this age. The best would be to just cut it off (I use small blunt nosed mustache scissors), it will grow back (sorry to say
). It's puppy hair anyway, they tangle & mat easily.
The cornstarch is the powder form. Just put some on the mat (doesn't matter how much, it won't damage the hair), work it in w/ your fingers. W/a slicker brush or comb, gently brush it apart. Make sure you're holding the hair closest to her skin so it won't get pulled. Good luck & let us know how it goes.