Mushin's Tower Trial Arena Unlocking

[xSimplify]

I was wondering if anyone knows how to unlock the Trial Arena in Mushin's Tower. I've unlocked it before on my other account but I can't unlock it on my new one. I've reached level 55 and it's still not unlocked. 

[Hanuku]

You need to read the letter for it. If I remember it's called "Trial Invitation" or something close to that.