Welcome to Justanswer.com, my name is Dustin.I'm sorry to hear about the issue you are having, I will do my best to help you resolve it.
If the forerunner hadnt been used in some time then what you may need to do is reset the device first, and then update it in order to get it to pair with your phone.
To get started, reset the forerunner heres how https://www.youtube.com/watch?v=xzPVkkysEDU
Once complete, connect the forerunner to your computer and then update it using the garmin express program. http://www8.garmin.com/support/download_details.jsp?id=6589
Once updated, download and install the new garmin connect app for your iphone, onto the iphone itself. https://itunes.apple.com/us/app/garmin-connect-mobile/id583446403?mt=8
At that point, you should be able to pair using the garmin connect app.
If you have any further questions, let me know.
I strive to provide great service, and greatly appreciate a 5 star rating clicked and submitted when you are satisfied.. I always answer follow up questions.Dustin S.Senior Technician and Network Switching Systems Operator / Maintainer.