If I attack with Isareth the awakener, and a have a creature in the graveyard that has a Morph ability, such as Den Protector, or Hidden Dragonslayer, am i able to pay the 'Morph cost" i.e. 3, and return the creature card from my graveyard to play face down?
Nope. A creature with Morph can only be cast as a face-down creature. Isareth's ability doesn't let you cast the creature card in the graveyard; it just gets returned to the battlefield, so it will always be returned face-up.
Commander/EDH Decks:
BRG The Blood of Jund - Kresh the Bloodbraided BRG
WR The Blades of Goldnight - Gisela, Blade of Goldnight WR