Try the Myst V installer from the link in my sig. I haven't tested on Mountain Lion since I don't have Myst V any more but it ought to still work.
(If you do prefer the Terminal method, the version of JAD that Heimdall linked to at the start of this topic is a PowerPC-only one and won't run on Lion or later. You'll need to download the Intel version from here, or use some other java decompiler.)
 Sorry, scratch all that. Myst V itself is PowerPC only and won't run on Lion or later. You'll have to play it on Windows or an older Mac OS X system.