The site and here have information about obtaining a visa for those traveling to the United States. Both sites have links to websites for U.S. embassies and consulates worldwide. The embassy and consulate websites have very helpful information about procedures, timelines, communities served, required documentation, and fees.

International registrants should be particularly aware and careful about visa requirements, and should plan travel well in advance. All visa inquiries must be handled by ACM HQ. Please send your request for a letter in support of a visa application to supportletters@acm.org include your name, mailing address, and fax number, as well as the name of the conference you are attending. (Authors of papers/posters should also include the title). Requests for letters are handled in the order they are received and it can take up to one week to receive a letter from ACM. Please note that ACM does not issue formal "letters of invitation" to any of its conferences.

See here for ACM's FQA on visa support letters.