Here is the US State Dept link http://travel.state.gov/travel/cis_p...s/cis_956.html

As you say, Visa not required for 90 days or less.

ENTRY AND EXIT REQUIREMENTS: U.S. citizens need a passport to enter Macedonia. A visa for Macedonia is not required for tourist/business purposes for stays up to 90 days. For stays longer than 90 days, American citizens need to obtain the appropriate visa at a Macedonian Embassy or Consulate prior to their trip