Did you get the modem connection information in writing? If not there is not much you can do about it - it's your word against theirs and they, of course, will deny it, even if you could find out who told you this.
On your second point about Norton, you say their website "seems to indicate". Well, I'm afraid this is how adverts are worded - to suggest the best possible deal or conditions. I've not seen the site in question but, have another look and check what it actually does say. It may be that on a subsequent reading, it doesn't seem so clear.
Sorry if the above comes across as me being on Dell's side - I'm not, I assure you but, what I am trying to indicate is that anything at all that you ask when buying things that involves buying extras should be confirmed in writing and also to read very carefully what the terms and conditions are on any special offers "given away" with your purchase. It may be, of course, you are right on both counts, in which case you should keep badgering Dell until you get a reply. I've heard that contacting the managing director of Dell UK and the CEO of Dell HQ in the USA can work wonders in sorting out problems but you should do it by snail mail and send it Special Delivery (to the UK address anyway).