Stuffit Expander should handle the tar and gzip files for you. Once you've run those through, you should have a folder called "dnetc-macosx-ppc Folder'. You're going to have to get into the terminal to actually run the program, as it's a Command Line program, and not a GUI based one like the old MacOS Classic was. The terminal is at /Applications/Utilities. From there, you need to navigate to where the folder containing the client is.
OS X uses *nix terminal commands, so here's a crash course if you're not familar with them:
cd - Change directory
ls - list all items in a directory
Now, when you start up terminal, you're going to be in your "home" folder. We need to get you to where the client is, which is most likely on your desktop(if the folder I first mentioned isn't on the desktop, move it there now). Now type "cd desktop", this will take you to your desktop folder. Now, type "cd 'dnetc-macosx-ppc Folder'", and note the single-quotes, they make cd understand that its all 1 folder name, and not multiple commands. Now, type "./dnetc", ./ is the command to run a CLI application, and dnetc is the name of the app obviously. The client will start up, and you will need to configure it like normal(please see the FAQs on our website if you're not familar with configuring it).
Once that's done, the client will exit as you save the changes. Now type "./dnetc -hide", this will start the Dnet client hidden, so there won't be any terminal output while it's running. To stop it at a later time, you'll need to type "./dnetc -shutdown".
If you want to avoid all this hassle, I'd reccomend getting "Crunching Cows" from
VersionTracker. CC is a complete GUi front end for the OS X Client, and includes a copy of the client within itself. It's much easier to configure, and seems to be just as fast as far as I can tell. If you're not familar with *nix commands, I'd highly reccomend using CC.