No, you can't determine your carbs by the number of calories. You have to determine how many carbs are in each serving of the individual foods you eat, as listed on the package/container.
For instance a serving of bread might have 12 grams of carbs listed on the package.
For help with the bar code issue, try asking your question in the Improve MedHelp Suggestions forum. The mods there should be able to help you. You can access that forum via the following link:
http://www.medhelp.org/forums/Improve-MedHelp-Suggestions/show/110