IIS logs are very easy to splunk, but you need to tell it what format the logs are in (since you can alter the log format). Here is an example for you.
In inputs.conf ($SPLUNK_HOME\etc\system\local\inputs.conf), add a stanza like this:
[monitor://C:\inetpub\logs\LogFiles\W3SVC1\*.log]
sourcetype=MSWindows:2008R2:IIS
queue=parsingQueue
index=msexchange
disabled=false
In props.conf ($SPLUNK_HOME\etc\system\local\props.conf), add a stanza like this:
[MSWindows:2008R2:IIS]
TZ = GMT
SHOULD_LINEMERGE = false
CHECK_FOR_HEADER = false
REPORT-fields = mswin_2008r2_iis_fields
TRANSFORMS-comments = ignore_comments
Finally, we need to define the two transforms in transforms.conf (which is in $SPLUNK_HOME\etc\system\local\transforms.conf) as follows:
[ignore_comments]
REGEX = ^#.*
DEST_KEY = queue
FORMAT = nullQueue
[mswin_2008r2_iis_fields]
FIELDS = "date","time","s_ip","cs_method","cs_uri_stem","cs_uri_query","s_port","cs_username","c_ip","cs_user_agent","sc_status","sc_substatus","sc_win32_status","time_taken"
DELIMS = " "
The format of the mswin_2008r2_iis_fields is taken from the top of the IIS log file. This is (hopefully obviously) for the default IIS logs from Windows Server 2008 R2. The location and format have changed from version to version, plus you can alter both the location and format on a per-host basis.
For more information on these configuration files, see the documentation - freely available at http://docs.splunk.com