Ok I find a way, but probably not the most easy way ..
Example of code with "xx.y" for the instance to control :
Modules.InDomain("HomeAutomation.ZWave").WithAddress("xx").Get().Command("MultiInstance.Set").Execute("Switch.Binary/y/255");
Program.RaiseEvent( Modules.InDomain("HomeAutomation.ZWave").WithAddress("xx.y").Get();, "Status.Level", "1", "ZWaveNode");
####
Modules.InDomain("HomeAutomation.ZWave").WithAddress("xx").Get().Command("MultiInstance.Set").Execute("Switch.Binary/y/0");
Program.RaiseEvent( Modules.InDomain("HomeAutomation.ZWave").WithAddress("xx.y").Get();, "Status.Level", "0", "ZWaveNode");