Item mapping in Magento is a two step process. Unlike mapping items in the Admin Console where items are mapped direct to a tax code, Magento introduces a third component called "tax class" which all items are mapped to. These tax classes need to be associated to a tax code for taxability. On a high level, tax class is created first by mapping it to a tax code and then an association is made on the item to the tax class. This is the exact order in which mapping needs to take place.
Sales > Tax > Product Tax Class > Add New
Enter a tax code and the name of the class in the most generic sense such as "clothing." Save Class.
Final step. Catalog > Manage Products > using the filter and locate the product in question.
Once the product is located click on it to drill down to view its attributes > select the appropriate tax class from the drop down menu
Mapping is now complete and the item should be calculated using the taxability behind the tax code that was mapped to it.