Okay, perhaps you need to export a QT file (not sure why, but maybe). If so, I’m not sure what the problem is.
If you don’t absolutely need it as a descrete element in FCP, do the following:
Save the Motion project.
On the top of the canvas window is the name of the project, with a little page icon on its left. Grab this so that it comes loose. Don’t release the mouse.
Apple-Tab to switch to FCP and drag it onto the timeline. Release the mouse. Render and be amazed (this is called Round-Tripping).
If you need to make changes, select the Motion clip in the timeline, right-click and select Open in Editor.
Make your changes and save the revised Motion project.
Tab back to FCP and the changes will be there.
If memory serves, this feature (Round-Tripping) works on FCP from version 5.0 on.
Good luck.
Ken