Alerting is available in the trial version. After 60 days you'll have the ability to convert to a free license which doesn't allow you to have alerts among other things. See link below:
http://www.splunk.com/view/SP-CAAAE8W
You can create an alert by creating a search and it is documented with examples here:
http://docs.splunk.com/Documentation/Splunk/5.0/Alert/Scheduledsearch#Set_up_scheduled_search_action...
Set up your email server here: