Senator Joseph R. Biden Jr., fighting to salvage his Presidential campaign, today acknowledged ''a mistake'' in his youth, when he plagiarized a law review article for a paper he wrote in his first year at law school. Mr. Biden insisted, however, that he had done nothing ''malevolent,'' that he had simply misunderstood the need to cite sources carefully. And he asserted that another controversy, concerning recent reports of his using material from others' speeches without attribution, was ''much ado about nothing.'' Mr. Biden, the 44-year-old Delaware Democrat who heads the Senate Judiciary Committee, addressed these issues at the Capitol in...