by editing the plugin live in your site and doing things like:
print_r($data)
exit;
You can see what the data is at any given point in the execution of the plugin. If it does nothing then you know an if statement is not returning true or the page you are on does not use the content plugin.
As for displaying the value in a page, you need something like loadposition, by refering to the file you can see how it pulls the article text and then replaces part of it with the code it needs.
$regex = '/{loadposition\s(.*?)}/i';
preg_match_all($regex, $article->text, $matches, PREG_SET_ORDER);
if ($matches)
{
foreach ($matches as $match)
{
$article->text = preg_replace("|$match[0]|", addcslashes($output, '\\$'), $article->text, 1);
}
}
This just pulls part of that plugin out, but it shows the concept.
As for saving the value, if you just saved that value in the database, have you not just saved it? You say manually so I assume you mean in phpmyadmin or something. Refer to JDatabase in that case. http://docs.joomla.org/Accessing_the_database_using_JDatabase