Registry.GetValue always return null

The statement of Jason is right, the operating system is the problem, the below code will help you to resolve.

RegistryKey localKey;
if(Environment.Is64BitOperatingSystem)
    localKey = RegistryKey.OpenBaseKey(RegistryHive.LocalMachine, RegistryView.Registry64);
else
    localKey = RegistryKey.OpenBaseKey(RegistryHive.LocalMachine, RegistryView.Registry32);

string value = localKey.OpenSubKey("RSA").GetValue("WebExControlManagerPth").ToString();