ToME: the Tales of Maj'Eyal

How does one go about unlocking the new Solipsist class?

You'll get a message the first time you go to the world map at level 20 or above - "the ground is shaking" or something like that. It doesn't happen for every character, so you might need to make a few.

Is there any specific race/class combination to unlock the new class or is it just pure luck?

Pure luck.

30% is not pure luck, it's reasonable chance

I presume by luck he just meant randomness.

Jup, thats what I meant.
Cheers

I think he meant player luck rather than character luck.

I must be the unluckiest dude in the world. Or not really, but still. This was my eight 20 lvl char, still no message about shaking ground.
Is it min 20, or 20+, I mean can the message pop up later on, or should it generate the very second you get to the world map after reaching lvl 20?

EDIT: 9:th ones the charm.

...and to answer the questions myself, yes, 20, yes instantly.

I got the message, but nothing has happened. Is there a specific place I have to go to for the mission to start?

I saw it along the southern mountain chain between Evala and Last Hope

What's a good level to do that at? I went in at level twenty and saw the "your heart is pounding" message.