Updates are only available through Impulse (SDC was discontinued more than a year ago, and standalone updates even longer).
If you're installing from very old retail discs, it's likely that the registry entries that allow Impulse to identify the game simply aren't there. If you use the Detect Application option in the menu (blue button at top left) you can make sure that Impulse is aware of the games, though you may need to add the correct versions manually to the registry after doing so in order to allow it to update without a redownload (Impulse cannot determine the version of a program when using the detect option).
