Double-click on "Install Undelete".
Alternatively, if you want to install Undelete by hand, open "Install Undelete" in your favourite text editor and follow the instructions. The install file is well documented.
Once Undelete is installed, open the Trash, select the Attributes menu and add the new columns in the Undelete section. You might need to close and re-open the Trash folder if it was already open when you installed Undelete.