With the 6th edition of the Guides, the first step in rating an impairment is to select the Key Factor, or diagnosis. The regional grids generally group the key factors into several categories to make it easier to find the appropriate diagnosis. In some situations, the condition you're trying to rate may not be listed and may be part of a more general category; it is also possible the condition may not be rate-able.
In the case of narrowing/bulging disk, it is not the narrowing or bulging that is rated, rather the resultant pain constitutes the key factor. The cervical, thoracic, and lumbar spinal regional grids all include a category of soft tissue and non-specific conditions which includes a condition of "non-specific chronic, or chronic recurrent pain".
You may also determine this from a process of elimination - narrowing/bulging disk is definitely not part of any of the remaining categories (i.e. motion segment lesions, fractures/dislocations, spondylolisthesis, etc.)
Once the key factor of spinal pain is selected, you can then further refine the impairment by applying any appropriate non-key factors.