Plugins: cannot find package jenkins.model

[Dumitru Daniliuc]

Hi,
I'm working on a plugin and I followed all steps at 
https://wiki.jenkins-ci.org/display/JENKINS/Plugin+tutorial. If I import 
classes from hudson.*, my plugin builds. However, if I import 
jenkins.model.Jenkins, I get a compile-time error: "package jenkins.model does 
not exist". Where do I get the relevant jar file from and how do I add it to my 
classpath?
Thanks!