I would like to suggest some alternatives to get your premises code.
First Alternative: As Mr. Ram Avtar Singh has told you can logon to www.aces.gov.in and then go to RET > Fill ST3 > Fill. There you can see the premises code.
Second Alternative: logon to www.aces.gov.in and then go to REG > View > Latest ST - 2. You will be required to give a range starting from date of subnitting ST - 1 and ending at some lated period, say current date. Click on the ST Registration Number to view ST - 2. Premises Code is given Under Address of Business Premises.
Above two alternatives will work only where ST - 2 has been obtained by the assessee. In case you have not filled up ST - 1, file the same & submit a hardcopy of ST - 1 alongwith the documents required with your range superintendent. Or if ST - 1 has been filed, but ST - 2 has not been issued by the Department, there may be some problem with ST - 1. Ask your Superintendent for the Deficiency notice and fulfill the requirements.
Third Alternative: Though this will not work for every assessee. Click the Link: https://cbec.nsdl.com/EST/AssesseeVerification.do Enter the ST registration Number. This will Give you your 6 digit location code ( Premises Code is Location Code + 4 digit alphanumeric code). In case the assessee has only one business premises try to add "A001" on the right side of the location code.
Fourth Alternative: Contact your Service Tax Range Superintendent.