Sorry to hear that you are having trouble importing video from your Sony HDR-CX550. I found an article that provides some information specific to your camera and iMovie '09:
For versions previous to iMovie ’11 v9.0.1, including iMovie ’08, ’09, and ’11 (v9.0 only):
To import the AVCHD content from the camera when MPEG-2 content has also been recorded, you must first import and erase the MPEG-2 content from the camera. You should then be able to import the AVCHD content.
I hope this helps!
If your video clips are on a SD card, put the card in a card reader and import from there. If you are attaching your camera via USB, be sure camera is plugged into AC (wall) power.
You just need to attach the card or camera and use FILE/IMPORT FROM CAMERA. You do not need to drill down to the individual mts files.