Why can't Earthland magicians use magic in Edolas without the X-balls? When Natsu, Wendy, Happy and Carla went to Edolas they couldn't use magic at first. Later Natsu and Wendy were given the X-balls to regain their magic usage. Yet even without the X-balls, Exceeds could use their magic. This is according to the wiki because they have their Magic located inside of their bodies just like the Mages of Earth Land. 


  Presumably, this is due to the fact that they are the only beings in Edolas whose Magic is located inside of their bodies, just like the Mages of Earth Land.


If both have their magic located within their bodies, then why are only the Exceeds able to use their magic, but Mages need those X-Balls?

Also related to this question, I was wondering why the Exceeds even lost their magic usage to begin with. It was said that they need to have the right mind-set to be able to use their magic, but when they entered Edolas for the first time, they didn't know magic was limited, so they were supposed to have the "right mind-set" at that time right? So why did they lose their magic usage when they arrived?
To answer your first question: because it's said so. (Possible explanation could be that exceeds were laid there so their bodies has some mechanism that they just needed to awaken but mages needed the X-balls for that. But this is just a speculation as we only know as much as it's said: mages needed the X-balls, exceeds didn't.)

To answer your second question: they were not supposed to have the right mindset. They were pretty much in doubt about everything especially about themselves. What are they, what are they for... what they knew so far changed quite drastically in a few hours of time. While it's not stated explicitly in Fairy Tail, in most of the other universes (including RPGs) using magic needs quite some concentration, so that "right mindset" thing can be more crucial than you think.