I found this on the Adobe support site ….
“Export to FLV” unavailable in QuickTime Pro or other video application on Intel-based Mac.
On Intel-based Macs the option to export to FLV will not be available in QuickTime Pro (or other applications that use QuickTime as their exporter, such as Final Cut Pro).
The Flash 8 QuickTime exporter plug-in (FLV.component) is not a Universal Binary. As a result QuickTime cannot ‘see’ it, and the export will not be available. This issue only effects Flash 8 and earlier. The Flash CS3 Professional exporter is a Universal Binary.
To resolve, set QuickTime Player to run in Rosetta mode:
Select QuickTime Player in the Applications folder.
Choose File> Get Info.
In the General section, choose “Open using Rosetta”.