It depends on the reason you are filing the Form 843.
Go to this website for the Form 843 and on page 3 Where to File is listed - https://www.irs.gov/pub/irs-pdf/i843.pdf
If none of the specific reasons for filing are for your situation are listed then you mail the form 843 to the IRS service center for California -
Department of Treasury
Internal Revenue Service
Ogden, UT 84201-0002
