Only iMovie '08 or newer supports the AVCHD format. Previous iMovie versions do not support AVCHD.
The computer requires an Intel Core Duo processor or better to support AVCHD video.
If Apple iMovie software is not installed on your computer, or you are trying to transfer 60p (1080 and 720) AVCHD video, which is not supported in iMovie, you can manually transfer videos to computer. Follow the alternative steps mentioned on the below support article to copy the video from the camcorder:
If both AVCHD- and MP4-formatted movies are recorded, the AVCHD movies may not be recognized. If this happens, import all the MP4 movies to the computer and then delete them from the camera. Afterward, disconnect and reconnect the USB cable so the AVCHD movies can be recognized.