The tools menu needs to be enabled with a special launch option, -tools. It may also benefit you to run the game with the -novid option so the Valve intro movie does not play. To set launch options, open the Steam Library, right-click on your Source game and select Properties. In the GENERAL tab, click on SET LAUNCH OPTIONS... and enter –novid –console –tools, as shown in the following screenshot.

This will cause the game to quickly boot into tools mode and you can then load the console.