Belerin's Codex Of The Planes 7

7

There is also the possibility that the powerful ancient magic spell of
"planar travel" could reach the Dream (as it could most locales).  However,
as the power of such ancient spells is currently finite in nature, even a
scholar who knew the spell might not be able to use it as the alatharya once
did.  

There are also rumors of very powerful spirit scholars, psionicists, and
even void scholars finding their way into the Dream.  Whether this is the
result of secret spells only the most powerful in their element has access
to, or a random result of delving into those forms of magic is unknown. 
Whatever is the case, the explicit conditions relating to such travel has
been kept a secret.