I thought Amex had pretty tight requirements on credit scores. To be honest, I would be surprised if they would give a regular charge card to someone with a 600 credit score.

I used to work at a bank, and if someone came in with a 600 credit score and wanted a car loan, we either denied them or charged an outrageous interest rate.

I'd suggest paying down your revolving balances. That will improve your score. Reapply once your score is in the 700s.