Modify the plugin “post teaser” with custom fields

hi, i am using post teaser plugin.

it work that when you want read the article you can click on the full permalink article to show all.

well i want link the permalink at a custom field of the article.
i read info and find the function

get_post_meta($post_id, $key, $single);

well i modify it at :

get_post_meta($post->ID, ‘wpo_sourcepermalink’, true)

where ‘wpo_sourcepermalink’ is the custom field.

but don’t work, i found the problem. if i write the exact id number instead write $post->ID it work. so the problem is there..

how can i do?

sorry for my english but it isn’t my motherlanguage