Can I add "MO:Operating Unit" profile value to my Profile value Lists using"Define Profile Options" if I do not have one? Can someone help me how to do it?

No you should not create a new profile option. The ability to create new profile option is there so that you could create new profile options to be used with custom applications.
You might want to elaborate why you want to do that? See if that particular profile option is hidden for some reason. You can make it appear again by changing display attribute but you should not create a new one. Each existing profile options is tied to built-in functionality of Oracle Applications. Creating a new one would not help resolve your issue.