@lusall,
I have Mac OS 10, version 10.4.11 and use the safari browser. I just went to my yahoo email account to compose an email, saw Show BCC in the space above the compose window, clicked that, and I could then add addresses to the BCC line. So, I don't know what might be happening on your email or your browser...
I guess this post of mine will be to bump the thread so another Mac user can see it. And welcome to a2k...