Why can't I start the Software-Sources Dialogue?

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http://askubuntu.com – I'm using Ubuntu 12.10 and I would like to install additional drivers, but as the title says, it's not possible to start the Software Sources GUI. The Menu Entry in the Ubuntu Software-Center Edit > Software Sources is disabled and not clickable: Can anybody tell me how I can start that Software-Sources dialogue? Hope anybody can help... Clemens (HowTos)