To join Diddlycraft, the first thing you must do is install your World of Warcraft client and any expansions you want to play.
Once you've done that, you need to upgrade your client to the same version being hosted by the Diddlycraft server.  That version is currently: 3.5.5a
Here's how you upgrade:
Inside the directory to which you installed the client is a file called realmlist.wtf  Make sure its contents are as follows:

Code set realmlist us.logon.worldofwarcraft.com set patchlist us.version.worldofwarcraft.com

Now, fire up your client and log in with a dummy name and password.  Anything will do.  I tend to use "nobody" and "nowhere".  This will get your upgrade started.  Unfortunately, this can take a LONG time, especially if you're starting from a basic fresh install.  Keep doing this step until your client is at the correct version.

Make sure you don't go past the specified version!  As of this posting we're at the latest version, but in the coming weeks/months I'm sure there'll be newer patches and installing those will leave you incompatible with Diddlycraft!

Now that you're running the right version, its time to edit the realmlist.wtf again.  You need to point your client at this server.  Change the contents of the file to:

Code set realmlist darkshade.homeip.net set patchlist darkshade.homeip.net

That's it!  Your client is ready to play!

If you haven't done so already, go to the Manager page (http://darkshade.homeip.net/wow/) to create an account.  This will be the account you use in the client to log in to the server.

Enjoy!

Oh, one further note.  When you launch the client, be sure you're executing wow.exe, not launcher.exe.  Right click your desktop icon and look at the properties.  If you use launcher, it may revert your realmlist file back to pointing at the official pay servers.