。
She approved of my plan.
Literal
She [topic-は] my plan [to-に] was-in-favor.
賛成する ('to be in favor of, agree with') takes its target with に, framing the plan as a direction the speaker leans toward. Distinguished from 同意する (which emphasizes consent or matching of opinion): 賛成 is the natural choice for proposals, plans, votes, and motions, while 同意 fits opinions and statements of fact. Together they cover the territory English flattens into 'agree.' 計画 ('plan') is the standard Sino-Japanese word; the more casual プラン (loanword) is also widespread.