Are you referring to logging into Splunk itself or an account on the OS created for Splunk to run under?
If the former, then you can reset the Splunk password by following the steps below (needs filesystem access):
If your looking for the latter (to login using the Splunk account on the OS) then you'll find you will not be able to login into the account that the Splunk installer created since it does not have a valid password defined since the account is only designed to be used by the local Splunk processes. The password for the splunk account in /etc/shadow is usually set to an exclamation mark which prevents anyone from logging in with this password.
thanks luke.