Please help. I bought a programmable thermostat which can be used for baseboard heaters. The heaters are 208/240v, the old thermostat is a low voltage wire(black and white). When I opened the heater, it has the 240v connected, but it has a transformer reading: federal Pioneer Chaudair ; model# n27-ert: 60hz; amps 12.5 n.I.; v. sec 24. What do I do to use the new thermostat?

For the sake of simplicity you have the wrong thermostat. Get the correct low voltage one.