As I understand it, Avid only supports 2 GB AVI exports? And Encore 1.5 and earlier won’t accept QuickTime reference files (Encore 2.0 supports QT reference files).
Try exporting to QT as a self-contained movie. Encore should accept that. If you have any trouble with that, export only the video as a self-contained MOV, and then the audio as AIFF.