It is actually an easy problem to solve. When you import an existing project into your workspace, you may face with such a problem since one of your jar files could not be found by the builder. That's why, some of the lines of codes are marked with red warning and try to warn you " I CANNOT FIND THE LIBRARY THAT YOU CLAIMED TO BE ON THE GIVEN PATH! "
Simply, right click your main project folder and go to properties. Then on the left menu find -> Java Build Path. Click Libraries tab and check the path of the jar file that most probably has a different path than the folder you have unzipped your project.rar. Click edit and give the right path. Your problem has been solved ;)