With Greeting Card Designer, the Place command is placing the signature.psd file onto the back of the card.
Photoshop CS4
For Mac users with Photoshop CS4 - using Version 4.5 - the signature.psd file must be located at...
Macintosh HD\users\shared\greetingcarddesignerV4\signature.psd
Photoshop CS5
For Mac users with Photoshop CS5 - using the CS5 version - the signature.psd file must be located at...
Macintosh HD\users\shared\greeting card designer\signature.psd