。
Many housewives use frozen foods to great advantage.
Literal
Frozen-foods [object-を] skillfully using housewives [subject-が] many.
じょうずに ('skillfully, well') is the adverbial form of 上手 ('skilled'). 利用する ('to utilize, make use of') is more formal than 使う. The structure ends in 多い ('many/numerous exist') predicatively: 主婦が多い = 'there are many housewives (who X).' The whole relative-clause-like 冷凍食品をじょうずに利用している modifies 主婦.