The Ogre plugin was missing from the last release.
This can be downloaded from:
www.inivis.com/external/OgreAC3DPlugin.zip
Unzip and place the two files in the plugins folder. Once you restart AC3D, the exporter will appear on the File->Export menu.
This is a windows only plugin. The Ogre .dll that this needs is already included with the recent 6.7.15 release.