If your retirement Plan's return/Form 5500 was not sent to you by ARS for electronis signature before the original due date, we automatically prepare a Form 5558, Application for Extension of Time, to exted the due date 2.5 months into the future.
There is nothing you need to do to initiate this action. There is only one extension of time allowed. There is no down side to extending, it is just for more time.
If your Plan's year end if December 31, 2017 the original Form 5500 due date is 7 months after year end, or July 31, 2018. With the extension, the due date is an additional 2.5 months later, or October 15, 2018.