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:
First check to see whether you have a wsadmincommands.conf in SplunkWAS/local/ directory.
If you do, copy and paste the line command (replace the password with your real password) in wsadmincommands.conf, and run it in the shell to see if it can return any result. If you can see the log, then try to do it without the password. Please post back if the screen return any error message .