The installaler of versions > 2.6.3 32-bit do not offer the option to install the mp4v2 muxer. 2.6.3 does, as does 2.6.7 64-bit.
My OS is Windows 8.1, but this should make no difference.
Use the latest nightly, if you want to keep up to date with inclusions in the experiemental version of avidemux (2.6.x branch).
Installer versions are only updated occasionally for this branch, and are not associated with stable releases AFAIK.
Confirmed
fixed with r9008