Under Photoshop CS and Photoshop CS2, PSD2FLA uses a slightly tighter unlocking and verification system. If you are attempting to unlock PSD2FLA with Photoshop CS or CS2 then you will need to do the following:

In Photoshop, go to the Preferences panel and find the Plug-ins and Scratch Disks pane, and from there find the legacy serial number control and enter your Photoshop serial number there. The legacy Photoshop serial number must be the same as the regular Photoshop serial number.