The Splunk for Fortigate app at https://splunkbase.splunk.com/app/1063/ is for Splunk 5.x or older and older devices. If you are running on Splunk 6.x, it is likely this app won't work or at least won't work without diving into the configurations and setup.
If you are using Fortinet FortiOS 5 devices and Splunk 6.x, you must use the Splunk for Fortinet FortiOS 5 at https://splunkbase.splunk.com/app/1899/.
Thanks
i will try this
i work with Splunk 6.1.2, So, can it be a problem of compatibility ?????
Yes, it likely is. See my new answer below.