Epsilon on "Custom Plugin Undefined Function Error"

I am building another plugin to add Disqus Commenting system to Zenphoto.

I have the plugin working but whenever I call it I get the php "undefined function" error.

The plugin is setup like:

class disqus_comment_form {

//options function

function printDisqusCommentForm {
//print stuff
}

}

I have tested that the class exists on the page with:

if (class_exists('disqus_comment_form')){//test}

But it still can't find the function unless I use:

$disqus = new disqus_comment_form();

$disqus->printDisqusCommentForm();

All of the other example plugins I have been looking through don't have to do this but I can't figure out what I have done differently.

I've probably missed something very simple but any help would be greatly appreciated.