hi,
I just tested on the VM3 with CleanMart and everything worked just fine.
Probably you did something wrong along the way. The process is so simple though, i don't know what went wrong on your website..
1- Create "String" type custom field, and enter "ontop" for the position. (don't touch any other parameter, leave everything as they are)
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2- Add this custom field to any product:
Attachment ScreenShot2015-02-14at2.48.12PM.png not found
3- The "Specifications" tab will be visible and shows the custom field:
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I only did those 3 steps. nothing more and I didn't encounter any problem.
Regards.