Mystcraft 0.10.2 is up and ready! :) This updates Mystcraft to work on Minecraft 1.5.1.
There are a number of updated features, some bug fixes, and a couple changes.
Comparators should work with Mystcraft inventories and crystal blocks, I made my commands work with command blocks, and hoppers work with my stuff, etc.
I also made it so that not writing in suns/moons or lighting controllers won’t add instability directly. This won’t affect existing ages.

Lots of other things were changed and fixed. See the change log (link below) for more info.

To recap: there is only one download (found at the bottom of this post); This file is the universal install for client and server; it no longer goes in the jar.
Mystcraft goes in the mods folder.

If you are still lost after 0.10, check out Direwolf20′s Mod Spotlight!

Download: [1.5.1] Mystcraft Universal
Check the Mystcraft Forums for updates and the change log.