Conditions based on Attributes

I have spent hours trying to create a condition where the flat rate type is determined by the choice of one of the attributes of the product type. For instance, one of the attributes is a term reference of single or pair. Thus, I would like to charge one flat rate for a single product and one flat rate if they buy a pair of the items. However, I can find no way of choosing the attribute choice as a condition that will work.

For some reason, the choices available under conditions will not link to the choice of attributes in the product types. Any ideas?