The US Embassy requires citizens of some countries to obtain a Visa to visit the U.S. for personal and/or business travel.
Diversified Communications (producers of Kosherfest) have been informed directly by the U.S. Embassy and legal counsel that a letter of invitation has no bearing on whether a visa will be issued.
When applying for your visa, be sure to provide a copy of your registration confirmation as proof of your intent to attend the show, as well as any promotional materials you have received verifying the exhibition is a valid event. A hotel confirmation is not the same as an event registration.
If you are exhibiting at Kosherfest, your booth confirmation and contract to exhibit can be attached to your application for a Visa as supporting documentation.
Please review information on US Visas at: http://travel.state.gov/visa/visa_1750.html.