ubercart module: field attributes usage
1: Download uc_field_attribute & install the module
2: active it
3: setup a term eg. named it as: size (or color, or you want it to be the attibutes for product)
4-1: go content type add this term refrence and,
4-2: then go admin/store/products/classes/sweatshirt/field-attribute at here, you will see size(that you add refrence term field) appear here, make "size" be selected
4-3: then you add a product (the product content what you add the size attributes is),you will find the term was working as term and attributes now.