As far back as I can remember, FCP hasn't liked AVIs. Somebody correct me if I'm wrong, but I don't remember any version of FCP being able to work with AVIs without rendering.
Convert the files to ProRes first, using Compressor. Be careful to choose all the settings to match what you shot (frame size, frame rates etc.). I don't know if Nikon has a plugin for FCP (as Canon does) that would convert the files with all settings intact. After all, Canon went with H.264 QuickTime, much more Mac-friendly than AVIs.
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