Hopefully this will help. We are using WAS v7, but with the splunk_forwarder_addon_was app. The instructions for installation are partly incorrect.
In /opt/splunkforwarder/etc/apps/splunk_forwarder_addon_was/bin you need SplunkWAS7.jar (which I got from the SplunkWAS app as it wasn't included in the splunk_forwarder_addon_was).
You also need a subdirectory called SplunkWAS7_lib containing com.ibm.jaxws.thinclient_7.0.0.jar and com.ibm.ws.admin.client_7.0.0.jar which can be pulled from the WAS installation.
In /opt/splunkforwarder/etc/apps/splunk_forwarder_addon_was/local the most important file that was completely wrong in the instructions was was_dmgr.conf. The error I kept getting was "Can't load cell information from was_dmgr.conf" so I decompiled the WAStest.class from in the SplunkWAS7.jar to check what it was looking for. It uses these fields:
[cell1]
port=8879
ip=<dmgr_ip>
username=<dmgr_user>
password=<dmgr_passwd>
truststore_path=../local/wastest.jks
truststore_password=WebAS
pmi_level=set1
was_jar_version=7
was_node_whitelist=
was_node_blacklist=
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