Heya,

You just need to complete all the storyline, hit lvl50 on the way and it will carry on unlocking the higher tier dungeons and trials as you go along.
Story line is easy to follow, but I somehow missed a trial somewhere so I had to go back, somehow find it and catch up lol.

FYI here is a list of the Trials and the minimum level you need to be for them.

BTW it all starts with your first trial at lv.20 with the main storyline 'Lord of the Inferno'

The Bowl of Embers (Trial) min LVL 20
The Navel (Trial) min LVL 34
The Howling Eye (Trial) min LVL 44
Cape Westwind (Trial) min LVL 49
The Bowl of Embers (Hard) min LVL 50
The Howling Eye (Hard) min LVL 50
Thornmarch min LVL 50
The Navel (Hard) min LVL 50
The Minstrel's Ballad: Ultima's Bane min LVL 50
The Howling Eye (Extreme) min LVL 50

Hope that helps
The Navel (Extreme) 50
The Bowl of Embers (Extreme) 50