Your route has a parameter called "id", whereas your method has a parameter called "userId". These need to match.
So either create a route, like:
routes.MapRoute(
name: "EditUser",
url: "Account/EditUser/{userId}",
defaults: new { controller = "Account", action = "EditUser"});
Or change your method to be:
public virtual ApplicationUser EditUser(string id);
Note that if you choose the first option, you need to put that call before the existing default one, because any URL you enter will match against the first route which matches it.