Caroline,
The DV port on the GS400 is underneath the battery behind a small black rubber cover. It is next to the S-Video connector.

See this article on capturing DV with a Panasonic cam.

Look in your manual to page 56 in a section titled "PC Connection with DV Interface Cable" for a picture of how the DV cable connects.

Thanks everybody. I have sorted the problem by buying the firewire stuff and hooking up that way. My partner's gonna be very annoyed that he was basically leading me away from the light in regards to his assertions that USB and Firewire are essentially the same. Sometimes knowing about computers (networking is his speciality) just gets in the way.

And thanks Guy for your clear description on the firewire location and manual stuff. I'd looked through the manual several times, but I hadn't used any of the afore mentioned keywords.

Anyway, thanks everyone, i'm left feeling very positive about this website (esp. compared to the shop I bought the camera from, and compared to the panasonic help people who were no help at all).

: )