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 'Runkeeper' button
Click on 'Allow' to accept the link between nike.bullrox.net and Runkeeper
The connection button should appear completely
And a new button with the 'Runkeeper' logo, should appear at the end of each activity line
Click on Runkeeper logo of running activity that you want to synchronize
When the synchronization is over, a green Runkeeper logo should appear