First, read over this:
http://www.kenstone.net/fcp_homepage/motion_text_in_fcp_spencer.html
Then, consider just putting the Motion project itself in your timeline rather than rendering a movie - this allows you to still edit the project if you need to, and when you render, it will automatically render to match your sequence settings.
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Mark Spencer
Freelance Producer/Editor/Motion Graphics Artist
Apple-certified Master Trainer
Author, Motion 4 from Peachpit Press
http://www.applemotion.net