Dark Souls only comes with Partial Controller Support on Steam. You'll have to use your keyboard to enter text in game.
Steam has information about this support on their Big Picture mode FAQ page:
Controller Support levels:
Any Steam game can be played while in Big Picture mode, though not all Steam games support the use of a controller. Currently, Big Picture supports the Xbox 360 Wireless Controller for Windows, the Xbox 360 Wired Controller, and the Logitech Wireless Gamepad F710. To determine whether a game is controller-enabled, please refer to the following tags:
Titles marked with “Full Controller Support” indicate that they can be downloaded, launched and played using only a controller. These titles do not require a mouse or keyboard on hand for any configuration or in-game setup.
Titles marked with “Partial Controller Support” indicate that mouse and keyboard may be required from time to time. A controller may be used while in game, however installation, configuration and overall user interface may only be optimized for mouse and keyboard.
The above information is only for Steam games, non-Steam games may react differently again, and will completely depend on the developers including the feature in their game.