In the template there is a tag which takes the contents of a component. An article is a com_content component. you are trying to put in a caom_mail_to component? The beauty is that they both plug into the same slot.
Now you can only ever have one component on a page. you can have lots of modules, but only one component.
you set which component is on a page by choosing it from the menu comands. (each menu item refers to a component.) generaly the componetsn are of the com_content type, and are articles, but in your case you are wanting to add a component called com_mailto? Asuming the component is installed, all you have to do is select the new butto0n when in menu item manager, and then select the mailto component type.
the tag that is being used in a joomla 1.5 template is:
<jdoc:include type="component" />
If on the other hand you are trying to add a module to the template, that is a different kettle of fish. Youy need to create an instance of teh module, assign it to a tag (which exiusts in teh template) then select which menu items the module will be published on. The tag in a template for a module is like:
<jdoc:include type="modules" name="module_name_place_holder" />
you can put more than one module into a single place holder.
If you already have this basic knowlge, pass on the details of this component, and we will see if we cant find you a better solution.