Go to the address https://nike.bullrox.net/en/my-account.html.
Fill in the form, and click on "Connect to Nike+"
On top of your activities, click on 'Strava' button
Click on 'Authorize' to accept the link between nike.bullrox.net and Strava
The connection button should appear completely
And a new button with the 'Strava' logo, should appear at the end of each activity line
Click on Strava logo of running activity that you want to synchronize
When the synchronization is over, a green Strava logo should appear