Have you tried to log on to the server as the AD user you want to use to run the SIA and then try to configure the SIA?
Btw. I found a post about a similar issue regarding a different software and this issue might be VM related (your server is a VM right?).
http://answers.splunk.com/answers/74258/windows-server-2012-splunkd-service-access-denied
You can try to do the "work around" fix in that post and see if that works for you too....
