"Restricted drivers" means drivers for hardware in your machine that is not open-source (in short). Like nVidia or ATI graphics drivers - they haven't released them open-source, and Ubuntu (like quite a few others too) makes a difference between them and open-source drivers; tries to prefer open-source solutions, and by default has only those software reposities enabled that contain open-source or "supported" software. There are other reposities as well, but they're disabled by default, and can be enabled if the user chooses to use software that is non-free or non-open-source, for example.
You can enable the reposities by opening System menu, then selecting Synaptic package manager from the submenu (whose name I don't remember..) When Synaptic is open, in it's menus you can choose to manage package reposities, and there enable those you want - all, if you like. In the newer versions of Ubuntu there should also be a direct icon for that same tool in the System menu's submenu Administration (if that was the sub-menu) - System->Administration->Package reposities (or something close to that).
Once you have enabled the other reposities (called "multiverse" or "universe" I guess), close the tool, then re-open Restricted Drivers and it ought to work. Actually I thought the RD tool would do it for you if you wanted..oh well.