I install Papyrus at here. So how to generate code using Papyrus ?
In order to generate any text artifacts from UML models in the Eclipse Modelling Environment (i.e. Papyrus, TopCased, etc.) you should use Acceleo which is an eclipse implementation of the MOF models to text transformation language, the OMG standard language for models to text transformations. It is a very well made technology but it could take some time to become familiar with it especially if you do not know MOF and the Model Driven Architecture.
QVT is another OMG language but its aim is models to models transformations (not models to text). It is therefore not the right answer to your question.