Cheapest way I can think of:
buy imovie 11
use that to import the AVCHD
navigate to events folder of imovie 11, where the imported movie sits as apple AIC codec
drag that aic file into imovie 06, where it will edit fine.
But of course, if you get imovie 11, you can just edit in that, though lots of us regard imovie 06 as still a wonder after all these years.