You can download the latest version of Quicksilver here.
Please download and extract the Things Quicksilver plug-in.
Double-clicking the Things Quicksilver plug-in will install it into Quicksilver if Quicksilver is already installed on your system.
Once the plugin has been installed, open the Quicksilver window, activate Text mode by entering "." and create a new to-do:
Then press the Tab key and type in "addto" to make the Action of the Things plugin appear.
Then you just press Enter ... and you're done :-)
Please restart Quicksilver and re-install the Things Quicksilver plug-in following the instructions above if the "Add to Things" action was not successful.
You can learn how to use the plug-in syntax for Things here:
Things plug-in syntax