CiviCRM API explorer menu is not visibile on Wordpress
Try clearing your CiviCRM cache CiviCRM >> Administer >> System Settings >> Cleanup Caches and Update Paths. 

Check if the Support menu is enabled in CiviCRM >> Administer >> Customize Data and Screens >> Navigation Menu

The api3 explorer link is

/wp-admin/admin.php?page=CiviCRM&amp;q=civicrm%2Fapi3#explorer