I get the impression that there was a typo and he meant buying a Myer card, then spending that on acquiring a M&CG card at the Myer store. Not sure whether that's possible, but it makes more sense since as you say, Woolies doesn't sell M&CG cards.
Thanks for the benefit of the doubt - I did think that WoW sold the same M&C G cards as I get at Coles.
Does make it somewhat harder if using WoW cards then BUT if you can get the discounted Coles cards then you can buy the M&CG cards at Coles to double dip on the Myer one Points for petrol as well as Myer One points for Myer purchases.
Even better if buying something from Myer for Duty Free (eg: iPhone, tablet etc) you get GST back on the 5% discount as well.