Get URL from Custom Fields Value?

I have a plugin that generates a custom field for every post, with the value of a certain url. Know i want to fetch this url and use it as a link elsewhere. I’ve been searching on the forum for the answer, but only came up with part of the solution.

The name of the custom field is: rssmi_source_link
The value is a url in this form: http://www.something.com

This is the code i figured out so far, but it only direct me to my frontpage when i apply it ?

<a href="<?php $field_name="rssmi_source_link"; $field_value = get_post_meta($post->ID, $field_name, true) ?>"

Hope you guys can help me