The Journeyman Project: Pegasus Prime is very much a Mac game and it therefore uses the full gamut of Mac resources it has available to it. This means it can be very hard to extract the data on a non-Mac system.
Here is an attempt to document some sane way for everyone to play the game in ScummVM. Much thanks to eriktorbjorn for his file list and his Linux extraction script.
Since this is different on different platforms, I shall cover the "big three" here.
For Windows you need to use either the HFVExplorer or HFSExplorer tools.
If you're using HFVExplorer, you'll want to dump the files according to this document. If you see M on a line, make HFVExplorer extract as MacBinary and as a "raw copy, data fork" for ones beginning with R. Note that you'll have to rename any file or directory with "/" in the name to have an underscore ("_") in its place.
For HFSExplorer, you can pretty much use the same instructions as HFVExplorer, except that you want to extract it as AppleDouble instead of MacBinary (using the "extract data and resource fork(s)" option).
Mac OS X
Extracting the data on a Mac is actually the easiest because a) you can use HFS drives directly and b) you can then run the game directly without changing any file names.
First thing you'll need to do is to enable hidden folders. Then merge the PP Data folders from all four discs into one folder on your hard drive. Any files with the same name are identical. That's it, you're done. You can also use the macbinary command line tool to make MacBinary versions of the files.
You'll need to have hfsutils installed on Linux. Then you should run this script provided by eriktorbjorn with this file list (pasted into a file called filelist.txt). It should be pretty self-explanatory to run beyond that, I hope.
Since StuffIt is a completely awful tool that won't let you extract any of the resource fork data on a non-Mac system, there's no sane way to do this. Instead, we have a nice zipped version of the demo hosted on SourceForge which you can download here. You only need the "PP Data" folder to play.