You can input an absolute path (i.e.: "C:\Documents and Settings\bob\file.dll", just remember that between double quotes ruby needs double blackslashes so the above should be "C:\\Documents and Settings\\bob\\file.dll") or a relative path which is the game's project folder.
API.call(["C:\\Documents and Settings\\aDLL.dll", "function name", 'ARGUMENTS', 'RETURN'], args)