Help! No matter what I do, I can’t get a Widget tab on my presentation page.

I’ve downloaded several different widget-ready themes. They load fine, except no matter what I do, I can’t edit or add any widgets, as there is no widget tab on he presentation page of my administrator account.

Can anyone suggest a solution? I’ve checked my webhosting folder, and there are function.php files in each theme folder, but no function.php file in the main folder. Is this the problem?

If so, can anyone explain to me an easy way to resolve this?

thanks.