[Typo3-shop] How to add new fields to the shop extension (like price3)?

[Mohamed Azzouzi]

Hi,
I've tried to add a third price to the shop extension (tt_products) by
editing the tow files ( ext_tables.sql and tca.php) as Kasper shows in his
video (ExtManager_episode2.wmv) but my changes doen't get saved!

I also tried the by following the "Creating a basic extension" tutorial, but
there is no "Start new" button as it says in the tutorial in "Extending the
extension with last name field" at the 2.nd. paragraph!

What am I doing wrong?

Thanks,
  Mohamed