Because of licensing restrictions, the trial version of After Effects CS5.5 and earlier didn't include some features that depend on third-party software components that were only included in the full version of After Effects CS5.5 and earlier.
These limitations are removed for After Effects CS6, which includes all effects and codecs included with the full version. See this page for details:
http://blogs.adobe.com/aftereffects/2011/04/improved-trial-version-for -after-effects-cs5-5.html
North America
Europe, Middle East and Africa
Asia Pacific