A simply mailform is already included in CMSimple. You have to do 2 steps to activate that mailform:
At the place, where the link to the mailform shall appear, you have to insert the call of the mailform link in the template.htm of the template:
<?php echo mailformlink();?>
In the most templates, the call of the mailform link already is included.
In the CMS configuration (Settings => CMS => Mailform => Email) a valid E-mail adress must be set, otherwise the mailform does not work, and the link to the mailform does not appear.