To build the plugin from source, Maven is required. Build it like this:
git clone https://github.com/jenkinsci/conjur-credentials-plugin
cd conjur-credentials-plugin
mvn clean install
As another option, you can use the latest .hpi found under the binaries folder.
When you have the .hpi file, log into Jenkins as an administrator. Then go to Jenkins -> Manage Jenkins -> Manage Plugins -> Advanced. In the Upload Plugin section, browse for the .hpi and upload it to Jenkins:
After installing the plugin, restart Jenkins: