For enabling Verbose Logging follow these steps -

Go to Central Admin and Click on Monitoring

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Click on Configure Diagnostic Logging

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Enable Verbose Logging as shown below – Here I have enabled Logging for Excel and PowerPivot

Set  Least Critical Event to report to the Event Log and Trace Log to Verbose as shown below and Click OK

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Further Ref -

http://technet.microsoft.com/en-us/library/ee210681(v=sql.105).aspx