Hello,
I wish to modify some mage guild's quests rewards for a mod i am working on, but I don't know how to access to the "original" quests (the one from bethesda) in the CS... may someone help me ?

(I hope my englsish is not too bad, I'm french)

If you click on Character -> Quests, all the Mages Guild Quests will start with a prefix of MGxx. But what are you trying to modify? In most cases the quests gives you a levelled item. Depending on what you want to do, you could end up modifying each instance of the levelled item, the levelled list, or the result box at the end of the quest.