I know this sounds silly: did you purchase a key for Twixtor? If so, you'll need to run the installer again and enter in the registration information to get rid of the cross.
If you've already done that, then if you are running FCP or Premiere Pro you'll need to get rid of any previously rendered sequences. You can do that by turning of Twixtor on the clip and turning it back on.
If you continue to have problems, write to us at firstname.lastname@example.org and let us know which application you are using and on which OS. It's also helpful to have a screen shot of the registration dialog of our installer, with the registration fields of the dialog box filled in (but before you hit "Install registered version" button)