Simplified rep is neither a separate module nor it needs a separate license. The reason of being grayed out is not sure but it may happen if u r in some other mode like sketcher etc. or u r in process of redefining a feature or creating a feature. just check it out, it doesn't cause problem normally.
Chric, Simp rep is use to creat diff assemblies by excluding/including parts so you can use only if you are in assembly mode. If you are in part mode then it will be grayed out. So please check it. Hope this will help you out to fix your problem. Regards, Vimlesh Patelia.