。
Her eye was caught by the dress.
Literal
She [topic-は] that dress [on-に] eye [object-を] fixed.
目をとめる literally 'fix one's eyes / stop one's gaze' is the idiomatic Japanese for noticing something visually — letting your gaze settle on it amid the surrounding scene. The に here marks the object of the looking, not the location of the eyes. Note that とめる written in kana leaves the kanji ambiguous between 留める ('fasten, keep'), 止める ('stop'), and 停める ('park'); for 目をとめる the relevant kanji is typically 留める, since the gaze is being held in place rather than halted.