Search() API throws Access denied exception

I'm trying to search the archives through IndexSearch2.Search() API which throws an UnathorizedAccessException

I found the following error message in the system event log after I invoke the Search() API
Search request refused because the user does not have sufficient permissions to search any folder in the archive.

User: TESTDOMAIN\Administrator
Required Permissions: Read
Archive: ex3user3
Archive Id: 10F7D1CBC20362245A8CD814C59D631B61110000ExchangeVault.testdomain.local

How do I changed the permission for Administrator to access the archive?

thanks
*Savari