Hi,
Are you sure that you have entered
"ontop"
For Custom Field's "layout position" ?
It is very important!!, those string custom fields show up in the specifications TAB, ONLY if you put "ontop" into "layout position" field.
Please follow the steps exactly how they are described in the TUTORIAL.PDF:
www.flexiblewebdesign.com/puremart/TUTORIAL.pdf
Look at the 10th page.
Once you do it correctly, the "specifications" TAB should be show up in the middle of "DESCRIZONE" and "PRODOTTI CORRELATI"
Regards.
Forum Support.