QS doesn't automatically take your selection as input. There's a trigger you can set up to do this.
Go to QS prefs >> Triggers. Select "Quicksilver" in the sidebar on the left. There's two triggers here, the first is a trigger to activate QS in text entry mode, the other activates QS with current selection in the command window. Set up the Current Selection Trigger by checking the box next to it, and then double clicking the blank/"none" in the space in the trigger column. This should slide out the info drawer where you can set your trigger keys. If instead of seeing the "Select Current Selection in command window" trigger you see "null", it means that your Catalog is not set to scan for selections. Go to Catalog up at the top of the prerence window, select Quicksilver in the sidebar on the lect, and make sure that, at a minimum, "Proxy Objects", "Internal Commands", and "Internal Objects", are all checked. Then go back to the triggers pane and set up your trigger (you may need to relaunch Quicksilver for the updated Catalog to fix the Selection trigger). The one thing that I can't get to work in Snow is that in the Preferences >> Extras settings there's an option to "pull selection from front application instead of Finder". I can't get that to work. Right now QS Selection trigger only works with selected Finder objects.
