Hi Jo-Anne,
I hate when this happens!! I have never had good luck with getting bubbles out of cured resin without a seam showing, but heres what I would suggest:
1. You need to find a drill bit the same diameter as the resin.
2. Use that drill bit to drill out the bubble.
3. If its not to tedious, you can try to polish the inside/sides of the bubble with a fine grit sanding tool (something that will also fit whatever you are using to drill with)
4. Pour another layer of resin.
Whenever I have done this, I still see where the bubble was, but Im a bit fussy. Im not sure its noticeable to others.
Im anxious to hear what others have to say as well.