i was playing tonight against a burn/stall i had dim. fissure on the field and my opponent activated gravekeepers servant. i procedded to attack with a monster .... he said that "since i could not discard a card from the top of my deck to the graveyard"(dimm fissure) that i could not attack. i said "i am putting it in the graveyard and dimmensional fissure is sending it straight out of play. he went on to explain that at nationals this question had came up and they had ruled that you could not attack !! i am here to get some type of verification or ruling thanks for the help in advanced !! :D

Yep, your opponent was right, you can't attack with Gravekeeper's Servant and Dimensional Fissure on the field. Sending a card to the grave is a cost, if you can't pay the cost you can't attack.

Yes, how are you sure it is a monster? you might ask... knowledge about the deck is unknown for a game state, so due to uncertainty you still can't attack.

hmm... interesting thanks for the help !!