If you are not a US citizen, US permanent resident or from a country that participates in the US visa waiver program, you will need to visit the US Embassy or Consulate in your country to obtain a temporary visitor visa (B-1/B-2).

For a listing of US Embassies and Consulates worldwide, go to http://usembassy.state.gov/ .
For information on visitor visas including the US visa waiver program, please visit the US State Department website at http://travel.state.gov/visa/temp/types/types_1262.html.

We suggest that you begin your application for a visitor visa as early as possible. Authors who are outside USA will receive a letter of invitation by email after their paper is accepted for review.

Travel information is available here