You will need to configure your sourcetype as "dspjrn:5". Preview is a new feature of 4.3, and is not supported by the iSeries app.
As a side note, you could add the following to your $SPLUNK_HOME/etc/apps/iseries/local/props.conf:
pulldown_type = true
After a restart, this will allow you to use Preview, and select "dspjrn:5" in sourcetype pulldown. This will not prevent you from receiving the error; however, it will allow you to view the events, properly parsed, in the preview window.