"I am using talktalk and I need to create an extra email address"
Have you created the new address on the TalkTalk website yet? If not then have a look here. Once the new address has been created you can add it to your e-mail client.
"...according to talktalk there should be an "add email" item in the lefthand colomn of windows mail but I can't see it"
Because you're running Windows 7, I'm assuming you're referring to Windows Live Mail. To add a new e-mail account, click the little blue button top-left - see here, click the "Add" button, choose "Email Account" then follow the prompts to add your new address.