Most plugins are set to appear only on certain locations/pages. Something that is designed for a sidebar may not work well at full width at the top of the page.
If you really need to put a widget somewhere else, you can edit the plugin code. Change the allow_template and allow_region functions to simply return true, eg:
function allow_template($template)
{
return true;
}
function allow_region($region)
{
return true;
}
Edit: I may have misunderstood, do you want a widget plugin to appear on a custom page plugin? If so, yes you'd need to edit the widget plugin to output on the specific page. You can do that in the output_widget function, like this:
function output_widget($region, $place, $themeobject, $template, $request, $qa_content)
{
if ( $template == 'plugin' && $request == 'cusotmpage' )
{
// plugin output goes here
}
}