题主这句话有点错误，原句应该是：Those who are able took advantage of the opportunities for success and happiness that present themselves.

这里的present代表（机会或问题）产生，意外出现 ，that present themselves做定语从句修饰opportunities，themselves指的是opportunities。在这里可以理解为意外出现的机会（有稍纵即逝的含义）。