Thank you both for your replies.
I had tried accessing the NAS from a 2nd computer and that too failed to display the 'Add-Ons' tab, so this issue isn't related to cache.
However, in the word of Victor Meldrew - "I don't believe it!"
I logged back on to the NAS after reading your responses and low & behold the option is now there.
I would swear (if it weren't banned on the forum) that it wasn't like that for the past couple of days.
The unit hasn't been powered off since I looked before posting. I had power cycled it a couple of times after I had got it p and running. Since yesterday I was copying some files across to the unit and it had paused.
So now I am really puzzled having done nothing to the NAS to resolve this.
With regards to the firmware which was installed when I set it up, if was 4.2.15. Once registered the first thing I did was to update it to the latest version as I know that 4.2.15 is unsuitable for the 3TB drives. This took most of Thursday to complete as it took 5 hours alone to reconfigure the drives once the new firmware had been installed.