When Quicken does not know which Quicken account downloaded transactions belong to, you can not tell Quicken they belong to an account that is already activated for downloading. You must first deactivate that account for downloading, to allow it to be considered a candidate for the downloaded transactions.
Also, if you are having problems with Discover downloads, and you're not already doing so; try setting up the account using "Discover Card" as the financial institution (which will give you "Direct Connect" downloads), rather than "Discover Card Account Center" (which will give you "Web Connect"/"Express Web Connect" downloads).
[Aside: It doesn't seem to me that your problem is the same as the original poster's problem. When in doubt, start a new discussion.]