VPN extension was installed into Plesk.
'Install key' button produces the following error:
Class "VPNKey_Common_UploadForm" not found
This is a product issue with ID #PPP-3785. The issue was completely fixed in Plesk 12.
Upgrade to 12, in order to use the 'Install key' feature.
As a work around, you may generate a new key by using 'Generate key' button, download it and install it on a VPN client you will use for connections.