Yeah, You can't use the effect of a dark world monster if you dicard it with zombie master as johan said.
If you don't know what a cost is:
A cost is what you have to pay to activate effects such as paying life points or discarting cards, it's usualy written at the begining of the description and it's not part of the effect. Some cards that make you discard cards as a cost are: lightning vortex, the tricky, raigeky breack, tribute to the doomed, zomebie master ect...
Dark world monsters say "when this card is discarted by a card effect" meaning that costs won't triger they're effects, you have use a card that has the effect (not the cost) of discarting cards such as: Dark world lightning, card destrucciton, morphing jar or dark world dealings.
I hope that helped you, your question has been answeared so I'll lock this topic.