Drop-Down or Single-Select fields are valid fields of the workflow application. The workflow rule needs to be based on values available in the select options and in some cases needs to be the internal value.
To use Drop-Down in Workflows, think about the object types that you have setup in your Mappings. For example, if you have a Contact Mapping, then any of the contact-based records are records you'll want to setup rules for. If, however, you do not have any Company Mappings, then you don't need to setup any rules based on these record types. In fact, if you did setup any rules based on company records and you don't have any Company Mappings, nothing will actually happen.