Here is good tutorial about the Carriers and shipping features in Prestashop 1.5 http://www.webhostinghub.com/support/edu/prestashop-15-tutorials/shipping/how-to-add-a-carrier-to-prestashop-15
And here is video tutorial:
If you have any problems with the carriers or shipping feature please make sure that you configured everything correctly.
Error in the front end during the Checkout process. If you see it please make sure that you configure right as you see in the tutorials above.