When you use Quicksilver, it usually has various entries for an application: the application itself and other entries (such as the application's catalog). Notice here that you have selected the "Xcode.app (Catalog)", not "Xcode.app", which is the actual application.
Try hitting the down key to open the menu that lets you select other matching options for what you have typed in Quicksilver (in this case "Xc"). You may find the actual "Xcode.app" entry, which will let you open Xcode.
If that does not work, the problem may be that Quicksilver is not looking for applications in all your drive's folders. To solve that, open Quicksilver preferences and go to "Catalog > Applications". If the checkbox next to "Find All Applications" is unchecked, then check it.