Sikkim is still a restricted area and one still requires a special inner line permit to enter Sikkim along with your Indian visa. You only require a valid Indian visa to visit Darjeeling, Assam and Meghalaya, whereas a special inner line permit is required to visit Arunachal Pradesh, Mizoram and Manipur. Foreigners are still not permitted to enter Nagaland and Tripura.
You may obtain the Sikkim permit from the Indian missions at your respective countries and have it endorsed on your Indian visa informing them that you will be travelling to Sikkim or on your arrival at Bagdogra airport where counter for Sikkim tourism office is located if not then at Sikkim tourism office located in Sikkim Nationalised transport premises in Siliguri which is about 25 minutes drive from the nearest airport Bagdogra. If you are still unable to get your permit to enter Sikkim for any reason, you may get it at the Sikkim-Bengal border (Rangpo). Maximum validity of Sikkim permit is 15 days and if required it can be extended at Gangtok, the capital of Sikkim. We shall however provide you with all the necessary assistance on your arrival to Bagdogra. You will require to have a passport size photo