Download and install the Video Editor for Mac on your Mac, then let's start the editing.
Step 1: Add AVCHD File
Click "Add File" button on the toolbar or "File> Add File" to load AVCHD files.

Step 2: Edit AVCHD video
At first, choose the AVCHD file you want to edit(crop, trim, effect), then you can edit the video:
Crop - Cut off the black edge of your video to let you enjoy your movie in full screen.
Trim - You can trim any segment of your clips.
Effect - Set the Video Effects: Brightness, Contrast, Saturation, etc.

Step 3: Choose a video file format for saving edited AVCHD files
Choose the file, click "Profile" drop-down button to choose the formats you need, then click "Browse" button to choose destination folder for saving edited files.

Step 4: Exporting(Saving the edited AVCHD files)
When you are ready to saving the edited videos, you only need to click "Start" button on the buttom of main interface to complete the editing task.
Original:
http://www.video.editor-for-mac.com/articles/how-to-edit-avchd-video-on-macosx.html
