How do I implement hook_page_alter() in a module? I'm trying to add a piece on content to the $page['content'] variable, but it's just not working and the examples are not helping me. My code is the following:

function my_modulename_page_alter(&amp;$page) {
    $page['content'] = $extracontent . $page['content'];
}


dpm() doesn't allow me to see the available variables. What am I doing wrong?
Try using hook_page_build(&amp;$page). hook_page_alter(&amp;$page), which is called directly after it, is meant for themers to use after all the modules are finished with their business.  

I just implemented function MODULE_page_alter(&amp;$page), and it worked in the module as expected.
Did you clear the cache so the module is registered?