First, attach your camera to the Mac via USB while camera is attached to power source (not batteries only).
Or if you don't want to worry about the camera, take the SDHC card from the camera and insert it into a USB card reader. (Then you don't have to worry about power for the camera.)
Open iMovie. It should recognize the card and open the import screen. If not, you can click FILE/IMPORT FROM CAMERA.
Look to the bottom of your import screen and click the button to create a camera archive. This way, you have a backup copy of your original AVCHD that you can always go back to. (And you can erase your card for further shooting).
Now, in iMovie, FILE/IMPORT FROM CAMERA ARCHIVE and point to your archive. You will import into an Event. When you import AVCHD into iMovie, it will be converted to Apple Intermediate Codec. For the reasons see here.