How is the crew rest area on the A350 accessed?
It looks like your image came from this article, since they have the exact same one.

According to Boeing, this is the most practical place, and Airbus has finally decided they agree.


  For the first time in Airbus aircraft –and after benchmarked[sic] Boeing´s configuration-, the A350 XWB’s crew rest compartments are in the aircraft crown.--All images and quotes from article linked above unless otherwise noted.


This image shows that the cabin crew's rest area is at the rear of the aircraft, while the flight crew's is up front. It also shows how they've shifted out of the basement and into the attic:


Image courteously, but unknowingly supplied by core77.com

Further down in that article they state why the crew rest areas are where they are:


  The A350 XWB crew rests, located overhead in the crown area, have no impact on revenues space – whether cargo or passenger – and contribute to the efficiency of the A350 XWB cabin.


It also explains how you get there:


  Entrances to these rest areas are located outside the passenger cabin, so they do not use up seat space. “This is the first time a crew rest area on an aircraft will have no revenue-generating impact,” A350 XWB Marketing Director Bausor said.


Unfortunately, I'm guessing that the crew rest areas do not actually have a skylight one might infer from this image:



I'm sure the day-shift crew would enjoy sleeping under the stars at night, though the night-shift crew probably wouldn't enjoy the bright sunlight while trying to sleep.