Why I don't see the add on app option via file at "Apps->Manage Apps -> Install app from file" for my Splunk Cloud instance?
You will not see this option on all Splunk Cloud instances. If you have a managed Splunk Cloud deployment, you should contact Support to get apps installed. See http://docs.splunk.com/Documentation/SplunkCloud/6.5.0/User/TypesofSplunkClouddeployment
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