。
She tried on the party dress.
Literal
She [topic-は] that party-dress [object-を] try-on-did.
試着する ('to try on [clothes]') is the dedicated verb for the fitting-room ritual — distinct from 着てみる ('try wearing'), which is more colloquial and broader. Department stores even label fitting rooms 試着室 ('try-on room'). The first kanji 試 carries 'try, test' (also in 試験 'examination', 試合 'match'); 着 is the wear-verb root. パーティードレス stacks two katakana loanwords to form a compound — 'party dress' transferred wholesale into Japanese. Such two-loanword compounds dominate domains where Japanese imported the activity along with the vocabulary (ファッションショー 'fashion show', ビジネスクラス 'business class').