Hi Helen,
From my experience additional packs tend only to be made available from the developers site. I've bought a couple of games where expansion packs wouldn't work because I'd not bought them directly from the developer however, this is not always the case. (E.g. Reflexives Ricochet Infinity and Chronic Logics Gish)
I would suggest that you contact the distributor you bought the game from and ask them about this. Also write to the developer and ask about the compatibility, explaining that you bought the game from wherever; they may either give you another version of the game (provided you can prove that you purchased a copy) or create a patch for you. If they say it should work and it turns out that it doesn't, you have enough evidence for them to give you a refund. (i.e. They said it would work but it didn't so you would be entitled to a refund. Big Fish games are a big and reputable distributor, so I doubt you'd have any hassle from them).
You could always just buy the expansions and if they don’t work ask for your money back. Most distributors do give a money back guarantee (though I've never returned a game, I have with applications and have never had a problem getting my money back.
I hope the above is of some help.
Regards
Stephen