See Backing Up and Restoring Encryption Keys:
- Start the Reporting Services Configuration tool, and then connect
to the report server instance you want
to configure.
- On the Encryption Keys page, click Restore.
- Select the .snk file that contains the back up copy.
- Type the password that unlocks the file.
- Click OK.
Error 0x80090016 is Keyset does not exist.. This can occur when the crypto service is stopped (unlikely) or when you don't have access to the keystore you're trying to access. That would likely be caused by manually changing the service to an account that does not have enough privileges to access the keystore. Did you change the service account using the SSRS tool, or manually in the SCM? The later is unsuported. If you did the former and you're hitting a keyset access error, you'll have to call product support to investigate.