Inboxes don't come for free. Have you added an account yet?

I should have mentioned that its a Mac computer. I've tried adding another account, but the message that comes up is that the password or address is wrong, and it isn't. I was able to download and install Postbox, and that one had an inbox, but unfortunately, I didn't like it. Thanks for your help, but I'm going to get rid of Thunderbird and stick with Mac's Mail, even though I don't much like it.