Issues with installing of plugins for Jenkins - plugins

Question

SOLUTION
After I defined the proxy settings, it started working. See the documentation: Jenkins Behind Proxy->How Jenkins handles Proxy Servers.
I want to set up my CI environment (hosted on a Windows 2008 Server) with Jenkins. I installed Jenkins from a binary using the Jenkins Installer and can call it in the browser with localhost:8080.
Now I want to install some plugins, but there are multiple issues:
When I try it via the Jenkins CLI (e.g. java -jar jenkins-cli.jar -s http://localhost:8080 install-plugin git):
$ java -jar jenkins-cli.jar -s http://localhost:8080 install-plugin crap4j
crap4j is neither a valid file, URL, nor a plugin artifact name in the update center
No update center data is retrieved yet from: http://updates.jenkins-ci.org/update-center.json
crap4j looks like a short plugin name. Did you mean ▒null▒?
Apr 04, 2016 3:32:46 PM hudson.remoting.RemoteInvocationHandler$Unexporter run
WARNING: Couldn't clean up oid=3 from null
hudson.remoting.ChannelClosedException: channel is already closed
at hudson.remoting.Channel.send(Channel.java:578)
at hudson.remoting.RemoteInvocationHandler$PhantomReferenceImpl.cleanup(RemoteInvocationHandler.java:360)
at hudson.remoting.RemoteInvocationHandler$PhantomReferenceImpl.access$700(RemoteInvocationHandler.java:319)
at hudson.remoting.RemoteInvocationHandler$Unexporter.run(RemoteInvocationHandler.java:420)
at java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source)
at java.util.concurrent.FutureTask.run(Unknown Source)
at hudson.remoting.AtmostOneThreadExecutor$Worker.run(AtmostOneThreadExecutor.java:110)
at java.lang.Thread.run(Unknown Source)
Caused by: java.io.IOException
at hudson.remoting.Channel.close(Channel.java:1163)
at hudson.remoting.Channel.close(Channel.java:1138)
at hudson.cli.CLI.close(CLI.java:329)
at hudson.cli.CLI._main(CLI.java:511)
at hudson.cli.CLI.main(CLI.java:390)
Since I could not resolve this
Well, I tried to install the plugins via the Jenkins GUI -- but the content of the tab "Available" in "Manage Pligins" is empty.
OK, last chance: manual installation. So I did it as in the docu shown: Went to the list of the plugins on wiki.jenkins-ci.org, downloaded the plugins, put them to $JENKINS_HOME/plugins, and restarted Jenkins (java -jar jenkins-cli.jar -s http://localhost:8080/ restart).
Totally there are 11 plugins I instaled (or tried to). For all the plugins have been plugin folders automatically created in $JENKINS_HOME/plugins (e.g. $JENKINS_HOME/plugins/git). But: Some of the plugins (Clover PHP, Crap4J, JDepend, Plot, Violations, and xUnit) are shown now in the liste ot the installed plugins -- and the other (Git, HTML Publisher, DRY, PMD) not.
Is it possible, that these plugins are installed and just not displayed? Why? How to check this?
How to get the plugins installed?