It's really easy So follow the link in the artists comments and download the file.
Then open up two windows. 'My Computer' and wherever you put the file you just downloaded.
In the My Computer window click on 'Local Disk (C: ) (Or whatever yours is called) then go to Program Files> Stardock > ObjectDock > Backgrounds > Non-Tabbed.
Then in the other window click on the zip file of the dock you downloaded. Then click on the 'Leopard Dock' folder. Then click on 'Backgrounds'. There should be a folder that says 'Leopard Dock'. Drag that folder into the other window that has the non-tabbed files in it.
Right click on your dock and select 'Dock Settings' when that comes up click the 'Appearance' tab. There should be OS X Leopard in the backgrounds drop down menu. Select that and your done!
Btw: Would it be possible to increase the position of the icons so that there is more space between the empty dock and the beginning of the icons? It would even look more like the real Dock. I hope you understand what I mean. I'm not that into skinning ObjectDock and maybe it's a limitation of the app itself, but it would be really cool if that's possible.