[WrongSizeGlass]

visionsnap,
You need to navigate to:
<your home folder>/Library/Scripts/Applications and create a folder named 'OmniOutliner' or 'OmniOutliner Pro' (depending on the version you purchased).

Once you've created the appropriate folder you should copy the scripts plugin into the folder.

Snow Leopard doesn't have AppleScript Utility anymore. You'll need to navigate to /Applications/Utilities and launch AppleScript Editor. Once you've launched AppleScript Editor you can see the plugins by opening 'Preferences' from the 'AppleScript Editor' menu and clicking on the 'plugins' icon in the toolbar.