You need to go into the "Optional Installs" folder on your Leopard disc, go into the "XCode" folder, and install the "Dashode" package.
Once installed, go into the "Developer" folder at the root of the HD, then go into the "Applications" folder, and Dashcode will be there.
Hope this helps.
There did it on purpose because it's part of the "Xcode Developer Tools", and those are never installed with the OS.