Learn Turkish: Yeterince çalışmadın. - You did not study enough.

Yeter by itself is either a verb (it is sufficient), an adjective, or an exclamation (like "That's enough!"). To use it as an adverb that modifies a verb—describing how much you studied—you must add the adverbial suffix -ince. This turns it into yeterince, meaning "sufficiently" or "adequately."

This lesson focuses on the "o kadar ... ki" (so ... that) construction. How could I express a similar idea using that structure?

While yeterince simply states that a sufficient degree wasn't met, you can use o kadar ... ki to emphasize the result of not studying. For example, you could say: O kadar az çalıştın ki sınavı geçemedin (You studied so little that you didn't pass the exam).

Does "çalışmadın" mean "you didn't work" or "you didn't study"?

It can mean either, depending entirely on the context. If you want to make it absolutely clear that you mean "study" in an academic sense, you can specify by adding the word for lesson: yeterince ders çalışmadın (literally: you didn't study lessons enough).

Where does the spoken stress fall in the negative verb "çalışmadın"?

As a quick refresher on pronunciation, in Turkish negative verbs, the stress usually shifts to the syllable immediately before the negative suffix -ma / -me. In çalışmadın, you should put the emphasis on the -lış- syllable: ça-LIŞ-ma-dın.