。
She took advantage of my ignorance.
Literal
She [topic-は] my ignorance [target-に] took-advantage.
つけ込む (also written 付け込む) is a compound verb pairing 付ける ('attach, apply') with the inward auxiliary ~込む ('into, deeply'), yielding the figurative 'to bore into, exploit, take advantage of.' It takes its target with に (人の弱みにつけ込む 'take advantage of someone's weakness'). 無知 ('ignorance, lack of knowledge') is a Sino-Japanese compound built from 無 ('without') + 知 ('knowing'). The whole phrase carries clear moral disapproval — つけ込む is not a neutral 'use' but a calculated, opportunistic exploitation.