Each attribute comprises at least three columns of data in the import file with these headers: CAttrImpID, CAttrCat, and CAttrDesc. Group these columns for each attribute in your import file and save the file.

On the Fields tab of the import in the Extension column, select extensions to group the columns for each attribute. For example, the first grouping would have 00 as the extension, and the second would have 01 as the extension.