You may need to encode from Adobe Media Encoder - open the AE project, select the compmyou want to encode and use the presets. Here's what Adobe says Encoder can do: P2 (MXF)
Note: MXF is a container format. Adobe Media Encoder can encode and export movies in the Op-Atom variety of MXF containers using the DVCPRO25, DVCPRO50, and DVCPRO100, and AVC-Intra codecs. Premiere Pro CS5 (5.0.2 and later) can export MXF files containing the MPEG-2 essence items that comply with the XDCAM HD format used by such systems as Avid Unity. The standalone Adobe Media Encoder can also export files in this format. For more information, see this link.