This should work:
int valueToUse = Convert.ToInt32(ConfigurationManager.AppSettings[intCurDay.ToString()]);
Other methods (not recommended - just so you see where your first attempts went wrong):
int valueToUse = Convert.ToInt32(ConfigurationManager.AppSettings[string.Format("{0}",intCurDay)]);
int valueToUse = Convert.ToInt32(ConfigurationManager.AppSettings["" + intCurDay + ""])