If you change the Advanced Importing preferences in iPhoto so that the checkbox labeled “Copy items to the iPhoto Library” is deselected, movies you import into iPhoto after that change will not appear in iMovie. Note: This issue only occurs with referenced movies that are imported to iPhoto while using iMovie ‘09 (version 8.0.5)

See more here:
iMovie ‘09: Referenced videos from iPhoto Library not displayed in iMovie Event Library

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