I don't know if I'm 100% correct on this, but:
It's a new account, of course it's going to bring your credit score down...
In my case, I jumped on board with Amex at a super young age (being freshly 18, they were the only bank/lender that would give me credit). Got a Platinum Credit card (not charge).
I spent a lot, and of course only used my Amex. Shortly after my credit score got SUPER awesome, I got a few other "whatever" cards from banks just for the sake of it, but then I jumped on board the Platinum charge bandwagon when I was 19.
Again, I think it brought my score down immediately, but then BAM again, my credit got really good, after using the card normally...
I could be wrong, but I think the charge cards are GREAT for long term credit score increases. I like to think it was my Amex usage that allowed me to qualify for my first auto loan finance (vehicle was $100,000).
Just use it normally, eventually it'll help more then if you never had it.