To add SSH key in the client area please click on the SSH Keys menu as shown below.
On the open screen click on the Add New button
Enter the Key name, then paste the generated key in the specified box. then click on the Create button to save.
Then go to your instance and click on the manage button.
Click on the settings tab and then select the Update SSH Keys menu option. Select the added key in the previous step from the drop then click on the Update button.
Congratulations!!! SSH keys will be updated on your next reboot.