Nancy77,
The names of the fields don't matter, it's their content that will determine if it's valid information or not. In this case, "appid" seems correct, as it's supposed to be a two digits ID, and this is precisely what it is.
The error you mention is most likely an attempt to use "ps_imbcd" as part of the ONECODE function. The way PrintShop Mail works, it expects multiple numeric digits as parameters, and once you have entered them all, it will use those to generate a string of letters. However, attempting to use a field that is already a string of letters will not work, because it will try to convert it to a letter string, which it can't so since it's already converted.
So what you did on the screenshot looks good, all that's left is to apply the PSMOneCode font to it. Alternatively, since you are on version 7, you could use the Barcode Object instead. It will automatically draw the barcode and will not require a font, so if you don't have the font, I would recommend that you use this instead.
Hope that helps!
Regards,
Raphaël Lalonde Lefebvre