Mail.php is called in Base_Action.inc.php. It is no where to be found after installing Base, PHP + Pear+ required components.
You might be missing Mail_Mime, see below:
Mail 1.1.14 stable
Mail_Mime 1.5.2 stable
Mail_mimeDecode 1.5.0 stable
Net_SMTP 1.3.2 stable
Net_Socket 1.0.9 stable
This error is received when the PEAR Mail package is installed without using the --alldeps flags. I updated the instructions in base_conf.php.dist to explain this step.