In the WebGUI go to Manager --> Searches and Reports. Either create a new Saved Search (I wasn't sure if you had already created one or not) or edit the search you want change the email notification for.
Under the Schedule and alert section, mark the check-box labeled Schedule this search
You should then see all of the fields available for you to edit.
Under Alert actions, find Send email, and mark the check-box that says "Enable"
You can then modify the Email subject, Email address(es), and what to include in the results of your email.
For more options and functionality see this post -http://splunk-base.splunk.com/answers/621/email-alert-subject
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