CS4 is the first version to work with AVCHD media. You'll have to upgrade.
In addition to upgrading to CS4, you might want to consider your computer. If it's more than a year old, you might have issues with AVCHD material. It is tough to edit and requires a lot of processing power. There are some threads on the optimum computer setup for editing at this LINK. Note: this link is to the "Tips & Tricks" sub-forum in Premiere Elements. I used it, because both articles (in the CS4 forum) are included. Each is well worth the read, especially if one is contemplating a new machine, or chaning the setup on an existing one. Be sure to follow both links.
The SANYO footage is not strictly AVCHD... it is MP4. Actually it's an .MP4 container, using AVC H.264 contents. Confusing, huh.
I had success editing this footage in Premiere Pro CS3 by using MediaCoder HD (there's a free trial version available) and transcoding the files to MPEG2 files. You can then bring the footage happily into your HDV project in CS3 and edit to your hearts content. Copy the files off your card onto your PC, then run them through MediaCoder HD
Download MediaCoder HD here: http://mediacoder.sourceforge.net/
If you get wierd artifacts from the transcoding, there are plenty of other file formats supported by MediaCoder HD which you could try.
Be sure to report back with your results!