The add appointment icon is disabled in Calendar How can I enable the + icon on Calendar in my iPhone so I can add an appointment to it?

Since the + icon is not enabled, I am not able to add a future appointment. I have to get on a Mac computer to do that.
If your iPhone is syncing with iCloud properly your add button will disappear until the system finishes syncing.  If the sync is frozen, you can toggle iCloud Calendars off in Settings>iCloud, restart the iPhone, and then turn Calendars on again to restart the OTA sync.  If there still is no add button sync iPhone with iTunes to restore the calendar app.  On rare occasions you might have to restore the iPhone firmware.