First, make sure your SDHC card is not locked. There should be a little switch somewhere on it. Move it to the unlocked position, and then try formatting it through your camera. From a Google search I found that to format the card you just have to do the following:
"You must put the camera in playback mode .....the green arrow...menu and then the 3rd screen" (Source:
http://tinyurl.com/2b8m28).
To format it on your Mac, you'll have to have a compatible SDHC card reader. In the same thread as above, however, it says Panasonic's card formatting software is not Mac-compatible. You may be able to do it with Disk Utility or some third party program.
If you try everything that I've mentioned and it still doesn't work, you may need to replace the cards.
Hope that helps