Some people who have downloaded Adobe Media Encoder with Premiere Pro CS6 as part of a Creative Cloud Membership subscription have a problem in which the MPEG2, MPEG2-DVD, MPEG2-Blu-ray, Dolby Digital, & MXF OP1a codecs are missing.
To fix this issue, install the Adobe Media Encoder CS6 (6.0.1) update. Choose Help > Updates to install the update.