The most common issue with MT-Notifier involves the integration of the subscription checkbox with the comments form. There are two common errors with this process. First, a sample checkbox:
<p><label for="subscribe">Subscribe to This Entry:</label>
<input type="checkbox" name="subscribe" value="1" /></p>
Note the name of the checkbox, and that there is a value. These things are important! If you do not have the name set to subscribe, the subscription form will not be able to find the value you submit, and will not create a subscription.
Somewhat related is the value. If there is no value, MT-Notifier will not see that there is anything to be done during the posting of the comment, and it will not create a subscription.
Another common (and similar) problem is that the name on the form fields are not set correctly on the standalone forms. Here is a form to subscribe users to a blog:
<form method="get" action="<$MTCGIPath$>plugins/Notifier/mt-notifier.cgi">
<input type="hidden" name="blog_id" value="<$MTBlogID$>" />
Subscribe to This Blog<br />
<input name="email" />
<input type="submit" class="button" value="Go" />
If the name of any of the fields is incorrect, then MT-Notifier will not be looking in the right place for the data you are submitting! To subscribe to an entry, you should use entry_id, for a category, category_id and blog, blog_id.
Also make sure that, even if you have spelled the name correctly, you have not included any extra spaces. For instance, a name of "blog_id " (note the space at the end) is not the same as "blog_id". If you get a message about an invalid or missing key, this is where you should check first. Make sure the field names are right!
If the name is right, make sure you use the correct ID. For the entry_id and blog_id, you can use the Movable Type template tags <$MTEntryID$> and <$MTBlogID$>;, but there is no such tag for the category ID. In that case, you'll want to use a template tag supplied by MT-Notifier for just this purpose, <$MTNotifierCatID$>.