Creosote is a brown tar like substance which is the product of incomplete combustion of wood smoke. It results from engaging the combustor too soon or trying to operate the unit when the combustor temperature is too low.
This material must be burned off the combustor.
To burn off the creosote, put the combustor back in the stove and burn the stove at a higher than normal temperature for 30 minutes with the combustor engaged and the by-pass closed. Repeat as necessary until the material can be removed.
Note: If the combustor is coated with creosote, it is likely that the flue pipe and chimney are also coated with creosote.
Be sure to have the stove's exhaust system cleaned before operating at these higher temperatures to avoid a chimney fire.
Be sure the combustor has cooled completely before attempting to handle it again.