Compression fittings also work for plastics, but you may need an adaptor. Take the line with you and ask the plumbing guy at home depot or lowes how to hook it up. If the guy knows and the parts are there, he'll fix you up. Trouble with this hook up (above) is that you can't shut of the RO without shutting off your sink line too. You'll have to add a second valve for just your RO line.