There are two ways:

[indent]1. Within both realms, mortal and magical, there are doors or portals that are monitored by say custom controls
Parallel to the way we travel overseas. One practically instant travels to desired city/town through a magical sort 
of black hole

2. Magical individuals whom know how to create a black hole portals, through spells, rituals, ect may do so at will. 
It would be easy to find how to do it, its just the means would be tough. Its usually hard to create a portal, even 
more so to make sure you end up in the exact place you want without exposure - that's why most just use above 
method. However, those individuals who can may need a type of permit. [/indent]