Private payroll processor ADP said Wednesday that private-sector employment increased by 135,000 jobs in May, up from 113,000 in April. Economists polled by Dow Jones Newswires expected ADP to report a 170,000 increase.Economists look to ADP’s report on private-sector payrolls to provide some guidance on the Labor Department’s highly anticipated jobs report for May, which is scheduled to be released on Friday.Separate data on Wednesday showed that activity in the service sector picked up slightly in May to 53.7 from 53.1 in April, the Institute for Supply Management said, coming in above economists' expectations for 53.5. A reading above 50 indicates expansion in the sector.The ADP and ISM reports come ahead of the Fed’s Beige Book released at 2pm Wednesday, which measures business activity among U.S. service companies.