Two likely scenarios come to mind:
- a user matching the declaration already exists on the system, or
- the catalog retrieved from the master does not contain the resource.
The user might already exist for any number of reasons, but a good one would be that you created him previously via "puppet apply".
The catalog might not contain the resource if the site.pp file you describe is not in the right place or has a different name. By default the right place and name is "/etc/puppet/manifests/site.pp", but that that can be changed in the config file.
John