The docks Background folders after extracting,go in here.
C:\Documents and Settings\"Your user name"\My Documents\Stardock\ObjectDock Library\Backgrounds.
For Zoomer dock:
Start up OD, right click on the dock and select Dock settings. In the configuration window that opens, select "Appearance" in the menu on the left. Then to the right in the middle you will see a "background" dropdown, use it to select the new background you have installed. it will instantly change on the dock as you select it. Then close the window when you have selected the Background you wish to use.
For Tabbed docks:
Start up OD, right click on the dock and select Dock settings. In the configuration window that opens, select "Tabs&Styles" in the menu on the left. Then to the right on the bottom you will see a "Style" dropdown, use it to select the new background you have installed. it will instantly change on the dock as you select it. Then close the window when you have selected the Background you wish to use.
Hope this is what you were looking for