Thank you for the question. The Downloads folder is under /Users/<your username> (ie - /Users/Ben). If you open that folder in Finder you should see the Downlodas folder and can drag it back to the dock. Once it's in the dock, right-click the folder and select Options -> Keep in Dock.
Ben: Your instructions are not clear to this dumb Mac user. I went into System Preferences/Accounts/ . . . don't see a "downloads" folder. Must be in the wrong place . . .
Ben: All it took was an hour: I figured it out by myself. If you have another dumb Mac user who asks you the same question: Give him these instructions.
1. Press the F (for find) key, while holding down the Command key.
2. in the upper right corner, type the word . . . Downloads
3. In the new window, go down the list to the Ds.
4. And there you will find the folder: Downloads
5. Drag the Downloads folder to the doc, between the Documents folder and the Applications folder.