Mercer, a global human resources consulting firm, said today that it is planning to hire 300 new employees in Urbandale for a variety of positions, including sales, customer service and operations, according to a release. The company will have an open house and jobs fair from 3:30 to 7:30 p.m. Thursday at 12421 Meredith Drive in Urbandale as part of the search for its new employees. Mercer and Marsh, both operating companies of Marsh & McLennan Cos. Inc., currently employ more than 1,000 people in Urbandale. The hiring is being driven by the growth of Mercer's offerings in the benefits landscape.