。
She was satisfied with her new clothes.
Literal
She [topic-は] new clothes [with-に] was-satisfied.
満足する (まんぞくする, 'to be satisfied') takes its object with に — the thing one is satisfied with. The ~ていた form indicates this was her state at a particular past moment. 満足 itself is a noun/na-adjective meaning 'satisfaction' that becomes a verb with する.