Completing krokotopia is only one way to get in to marleybone, the other way is to have a friend that can get there and when he is there try porting to him, a loading icon will show up on the screen and when it is done loading or almost done, you can port to your friend, that is how i got to mooshu before i had completed the right quest.
If you just want to go to Marleybone to look around, you can teleport to somebody who has access there. The Troll's Ear Pendant (usable by Level 10 and higher) is sold in the amulet shop there. However, you can often get this cheaper at the Bazaar, when it's available.
