How can I connect a wireless thermostat?

Hi! I have a vaillant turbomax 824e combiboiler and I have bought a wireless Salus programmable room thermostat model rt500rf.I don't know where to connected inside the boiler.I am builder so I fill confident to connect it to the main and to the boiler if I could know where.On the thermostat instruction says that there are 2 options:230v application,and volt free application.what is the difference and which application I should use?Anybody can tell me,please,where to connected inside the boiler?Thanks!Tony