Add messeage text area to the checkout

-- url of the page with the problem -- : www.gobbe.se
-- HikaShop version -- : latest
-- Joomla version -- : 2.5.17
-- PHP version -- : 5.3.x
-- Browser(s) name and version -- : Google Chrome
-- Error-message(debug-mod must be tuned on) -- : N/A

Hi, I need to add a text area to the check out so a customer who choose to pick up products at store after an order is placed can send us a message when he wants to pick them up.
I thought I managed this in the Display => Custom fields but after I added a text area there it never appeared at the checkout...
So if someone knows where to do this please let me know.
Regards
Micke

Ok thanks, everything is clear in teh documentation