Same thing happened to me when I went from dialup to adsl. I had to buy an ethernet card and fit it. No big deal and I'm no computer geek. I just went to my local computer shop and told them I'd gone to adsl and told them what sort of router I had, they provided the ethernet card (wasn't expensive). I just put the ethernet card in a vacant slot and cabled up and voila, all done.
I was glad I went to the shop and asked for advice though, told them I didn't have a clue what I was doing but they were very helpful.
As stated above, this is definitely not a techie answer, just my experience.
btw I have a pc running Windows xp, no idea how to handle this if you have a Mac.
On edit: I just noticed your concern about being signed up to something you can't use. That shouldn't be the case. Your computer should be able to accommodate the ethernet card. As I said, they are not expensive and easy to fit so no need to worry that you have signed up for something you can't use. And on that can your broadband provider assist with some technical advice? Do they have a help forum on their home page?
Have a look here for some advice.
http://www.st-andrews.ac.uk/its/resnet/desktopcard.shtml