She didn't like using a brush pen.

Literal

She [topic-は] writing-brush [object-を] use [subject-が] was-disliked.

毛筆を使うのがいやだった — のが makes the verb clause the subject of いやだ ('disliked'). 毛筆 (もうひつ, 'brush pen') is the traditional Japanese/Chinese writing brush.