Each OverDrive system will have the Client software and Plugin software available for that specific version of OverDrive. The download may be in a different spot depending on the overdrive version you are running.
To download the version you need:
Pre V21 - Type the IP of the OverDrive server into a web browser connected to your network to find these downloads.
Version 21+ - Login to the overdrive server default credentials user: overdrive password: password, select the download tab