Hello, Jin.
Within the app, there is a Help menu and under that a Documentation menu item. If you select that, it should provide some guidance. Additionally, the Documentation from the app page on the Splunk App portal (http://apps.splunk.com/app/1629/) will provide the same detail.
The short answer is that you need to generate a client certificate from the Sourcefire Defense Center. This can be done from the System > Local > Registration menu. Once there, select the appropriate events from the left, then hit Save. Next, use the Create Client button to generate a client certificate. Download that certificate and place it on the Splunk server where the app resides, and put the path to that certificate in the box you're asking about.
Best of luck,
Colin Grady
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