Installing a plugin from the Update center results in:
Checking internet connectivity Failed to connect to http://www.google.com/. Perhaps you need to configure HTTP proxy? Deploy Plugin Failure - Details hudson.util.IOException2: Failed to download from http://updates.jenkins-ci.org/download/plugins/deploy/1.9/deploy.hpi
Is it possible to download the plugin and install it manually into Jenkins?
Yes, you can. Download the plugin (*.hpi file) and put it in the following directory:
<jenkinsHome>/plugins/
Afterwards you will need to restart Jenkins.