Whoops.
Looks like I changed the wrong string, producing errors with existing “Flat” symbols. Should be fixed in this build. You might need to perform a bit of cleanup to repair altered saves. Sorry.

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 on the 0.10 series, check out Direwolf20′s Mod Spotlight! Direwolf20′s Mod Spotlight for 0.10.4.00
If you are having trouble with writing, check out Direwolf20′s Writing tutorial!
Note that there are a couple changes between the video and the current mod, particularly that colors and things are easier to write. If you follow the tutorial, you’ll do great.

Download: [1.6.4] Mystcraft Universal 0.10.12.01Adfly Link
Download: [1.6.4] Mystcraft API 0.10.12.01Adfly Link
Check the Mystcraft Forums for updates and the change log.

Enjoy!