1. Make sure that you are in the New Gmail Version by using a Supported browser
list supported browser click here
2. If you using Google Apps for your domain make sure that you have enable the new service and new release click here
The Gmail Labs will not show if you using a older browser such as IE 6 or Firefox 1,& 2 Safari 1 or if you using the older version of gmail.
3. Check to see what version of browser you using click here