it should state:
".... use the service task and just call if from within the process..."
anyway in 5.4 I am afraid there is not web service task support in web designer so that mean you need to either use newer version or do the changes manually in bpmn2 file (most likely not appealing task). You could try to use this designer branch to build as it should be compatible with 5.4 but with new features. That branch seems to have support for Web Service Service task
HTH