CALL FUNCTION: Simple Post Thumbnails

I’m using:
http://wordpress.org/extend/plugins/simple-post-thumbnails/installation/

In the installation instructions it says:
Add the p75GetThumbnail(int $postID, [int $width, [int $height, [string $fileType]]]) function to your theme to get the URL of a thumbnail and p75GetOriginalImage(int $postID) to get the URL to the original image.

How do I add these to my functions? Just copying and pasting them in doesn’t work, I tried referencing http://codex.wordpress.org/Function_Reference/add_action
but I still can’t figure it out.