(.)。
I don't know how to use art files (.art).
Literal
Art-file (.art) [genitive-の] usage [subject-が] do-not-understand.
A basic software-support question. The structural point here is the particle on 使用法: it takes が rather than を with the verb 分かる ('to understand, to be comprehensible'). This is because 分かる is one of the Japanese 'state-like' verbs that treat their apparent object as a subject — literally 'the usage-method is not understandable (to me).' The same pattern applies with 好きだ, できる, 見える, 聞こえる, etc. Classic early-intermediate confusion for English learners. The file extension (.art) in parentheses is treated as an apposition to the noun アートファイル, clarifying which kind of file is meant.