So the Forge PR I’d made went through pretty quickly, but I spent some time after that trying to improve the system overall. It should do lots better in terms of memory churn, though the footprint is still a bit large.
There were also some general features and things added. I added a Gas block type to the grammar, so those will be handled correctly. Some existing ages with lakes or oceans of gas “fluids” may exhibit strange behavior, though. Mostly they are likely to fall back to their respective default blocks and the modifier symbols they had are likely to become instability, but you probably didn’t use those ages for much anyway. ;)
I also forced Minecraft to correctly unload Ages. This is a fix for ages only, so it won’t affect other dims, but it makes me happier.
Check the full changelog linked below for everything and let me know in the Mystcraft Forums what you think an if you find any new issues. :)

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 0.10.11.00Adfly Link
Download: [1.6.4] Mystcraft API 0.10.11.00Adfly Link
Check the Mystcraft Forums for updates and the change log.

Enjoy!