basic BuildWrapper problem

[Max Spring]

Having troubles with a BuildWrapper: It does not become active for any job.
I'm building against 1.438.
The plugin has also a Builder which works fine.
I had BuildWrappers working before (in a different context).
I thought there's no extra "activation" necessary.
Looked into the BuildWrappers of several other plugins, but can't see what I 
might be missing.
Thanks!
-Max

package org.example;

import hudson.Extension;
import hudson.Launcher;
import hudson.model.BuildListener;
import hudson.model.AbstractBuild;
import hudson.model.AbstractProject;
import hudson.tasks.BuildWrapper;
import hudson.tasks.BuildWrapperDescriptor;
import java.io.IOException;

public class MyBuildWrapper extends BuildWrapper{

  @Override
  public Environment setUp(AbstractBuild build, Launcher launcher, 
BuildListener listener) throws IOException, InterruptedException {
    System.out.println("MyBuildWrapper.setUp "+build.getDisplayName());
    return new Environment(){
      @Override
      public boolean tearDown(AbstractBuild build, BuildListener listener) 
throws IOException, InterruptedException {
        System.out.println("MyBuildWrapper.tearDown "+build.getDisplayName());
        return true;
      }
    };
  }

  @Extension
  public static final class DescriptorImpl extends BuildWrapperDescriptor {
    @Override
    public String getDisplayName() {
      return "My Build Wrapper";
    }

    @Override
    public boolean isApplicable(AbstractProject item) {
      return true;
    }
  }
}