Whoops. Looks like I made a mistake in my 1.6.4 update.

A critical bug was found in 0.10.8 where I incorrectly updated some MCP function names. This caused block modifier symbols to have the wrong identifiers. Symbols found on the update are wrong, and old worlds will have issues. This update addresses the latter, invalidating any block modifier symbols found since the update (which I expect to be relatively few…).
Since I was in the middle of a (pretty cool) new system, that’s included in this build… It should work, but if you notice anything odd with weather or biomes in ages, be sure to bring it up on the Mystcraft Forums.

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 0.10.9.00Adfly Link
Download: [1.6.4] Mystcraft API 0.10.9.00Adfly Link
Check the Mystcraft Forums for updates and the change log.

Enjoy!