PortalSettings is a class type and AdministratorRoleName is a property of it. You will need an instance of PortalSettings to access that property. Modules inherit from PortalModulbase, which has a Property PortalSettings of type PortalSettings, and this property is set when the module gets injected.

The code you are showing resides in its own class ModuleSecurity. ModuleSecurity  is not aware of the current context, it has no property or field PortalSettings. Luckily the DNN framework offers some libraries to support the developper. DotNetNuke.Common.Globals is a vb.module that contains a public function GetPortalSettings(), which returns an instance of the current PortalSettings.

Hint: make a right click on unknown objects inside Visual Studio and choose "Go to Definition".

VB allows to be lazy: GetPortalSettings.AdminstratorRoleName is the same as GetPortalSettings().AdministratorRoleName is the same as 
DotNetNuke.Common.Globals.GetPortalSettings().AdministratorRoleName .

As Globals is a VB module, it extends the set of available function and procedures. C# doesn't have a similar concept, you can not write this kind of libraries in C#.