When you create a symbol in the library go into its “Linkage” and change its “Class” to the path of where the class file is located relative to the fla. Separate each directory with a “.”
Once that is good, your class file should have a
package class.path.similar.to.the.one.in.the.library(without the actual class name)
public class ClassName() extends MovieClip
public function ClassName():void{
}
}
}