adatapost and SharpJohnnyG are both correct, depending on what you meant. Can you clarify your question. Are you trying to alter the text that appears above each page (eg "tabPage1") or are you trying to place text inside the tab page itself.
If its the first, then what SharpJohnnyG said is correct, you need to alter the text proeprty of the pages not the containing control. Either click on the page itself then alter its text in the property window, or click on the container and then access its TabPages Colelction from the property window (see image tabpages.JPG).
If you want to add text to the body of the page then (as adatapost said) the tab page is a container, you need to add controls to it. So to add text you can add a label or textbox to the page.