To access to your JCS (Java Cloud Service) environment, you can use Putty.
You need to configure a ssh session with a private key.
You must use your Public IP address of JCS Service, you can find it on your JCS account.
Port in 22.
The user is always "opc"
Set the location of your private key.
[Oracle Cloud & SSH: How to generate private and public keys (pub & ppk)]
Remember: You have used the public key during the creation of JCS service.
Now you can access to JCS.