That's right. I deliberately identified the site as "Parked" on the "Manage Domains" page, but didn't realise that this would mean that I couldn't upload to it. (Makes sense, I suppose; if it's parked, it's not being used.)
I tried that and, sure enough, the domain appeared on my FTP area. I've parked it again for the time being and will make it "Fully Hosted" again when I'm ready to go live with it.
Thanks very much for your help.