The wiki for it says near the spawn. Check your book in the thing that tells what properties and if it has them it should say.
They can be incredibly rare though; I've genned... 36 ages and still don't have the star fissure symbol.
I've started a new mystcraft game and i'm having trouble getting all the symbols i had on my first game (i decided to start fresh after i disabled "redpower world" to remove volcanoes and such, since it gave me bugged Ages).
For some reason, i'm getting a lot of cave worlds, i still don't have the glowstone symbol, but i found a star fissure once. Oh and it seems that mystcraft is doing weird things with thaumcraft, because i'm getting way too many chests in cave-world lakes. Also my flatlands tend to generate lots of monoliths, or maybe they're just more obvious because of it, dunno.
I can't wait to hit those huge wood tendrils with the axe of stream, although i'm guessing it'll break in one go.
What else... oh yea finding chickens can be a pain in the ass. I think i'll eventually start a farm and breed them but until i get there it's annoying.
Also i'm not a big fan of the checkerboard biomes, i find it ugly and it doesnt make sense to me.
But yea otherwise i'm doing ok, and i can't wait to get started making my own ages and play one mod in each