Here is some software description, to head off possible confusion.
-- URU: Ages Beyond Myst
The original project to create an online Myst experience. The online experience was cancelled, and this was released as a single-player offline game by UbiSoft in 2003. The game uses the HAVOK physics engine, so it is not considered portable from Windows to Mac. (Crossover might fix that, but I'm not aware of anyone that tried.)
-- Uru: To D'Ni
-- Uru: The Path of the Shell
Expansion packs for the Uru: Ages Beyond Myst game.
-- Uru Complete Chronicles
The compilation of Uru's materials: Ages Beyond Myst, To D'ni, and Path of the Shell, in one package released by UbiSoft in 2004. Still available for sale.
-- Until URU
An open-source version of the online game, meant to keep the URU community together in 2005. This was primarily Linux based, and meant as an online game with a local client. All public servers have been shut down.
-- Myst Online: Uru Live
The resurrected online Myst game, released by Cyan Worlds and Gametap in 2006. The Mac client software for this runs under the Cider (Cedega) engine, so no Crossover work is necessary on this version.
Cyan Worlds is working to add all content from the previous URU games into Myst Online. As of this writing (July 2007) there is substantial content from Path of the Shell / Complete Chronicles still to be added, but prospects look good to have it all in by early 2008.
Thus while some people (like myself) are still hoping for some conversion work on the single-player versions, the desire for it is shrinking rapidly.