the AVCHD is probably stored in a quicktime container. You either need the right quicktime codec or you need to transcode your media to codec that FCPx understands...
Check this thread too:
Post some screen shots of what you're doing. If you have the card or a camera archive and open the import window you should see the material in the camera portion of the sidebar. You only need to select the folder. You do need to go inside it.