Currently, the Java client api does not have proper support for Multibranch projects. Since these differ a bit from other types of jobs, how to use the Java client api's are not intuitive. The code base does not really take Multibranch projects in to account either. It would preferable if these could be supported out of the box with proper object models.
The current workaround is to mimic the Multibranch job as a folder, such as:
FolderJob multibranchRoot = new FolderJob(createdJob.getName(), jenkinsUrl + '/job/' + createdJob.getName() + '/')
def job = jenkins.getJob(multibranchRoot, 'master')