If you mean they don't appear in the "Render As" dialog, you can check to see if "Favorites Only" or "Match Project Settings" are check-marked under "Output Format" in that dialog. I wouldn't think these would be selected by default on a new installation, but it might be worth checking.
Thanks for your response. Would that it were so simple, but no: the AVCHD options simply do not appear in the list. I should say that this is a Windows 8 machine, though I don't know why that would affect this..