America Online Inc. laid off about 90 people from its advertising sales force yesterday as senior officials prepared to present a strategy to overhaul the struggling online unit to AOL Time Warner Inc.'s Board of Directors tomorrow. Before the cuts, AOL employed about 600 people in ad sales. The layoffs, primarily affecting employees at the online unit's Dulles headquarters and in New York, involve those who sell ads for display to users in individual cities or regions. The duties of the local ad force will be shifted to others in the ad sales division, known as the Interactive Marketing Group, sources said. That division cut about 100 employees earlier this year.