The 10.5 Server Admin tool is not compatible with the older versions of Mac OS X Server. If you were still running Tiger, had installed the previous Server Tools and upgraded to Leopard, they would still run, but you cannot install the older tools on Leopard.
You can always use the built in Leopard Screen Sharing tool to connect to the older server installations. I use it to administrate a Tiger Server from my Leopard desktop. Look in /System/Library/CoreServices/ and you'll find the Screen Sharing application to use. If you have Apple Remote Desktop, you could just use that too!
Michael