You should do something like this:
<?php $my_user_fields = slt_cf_all_field_values( 'user', $user_id ); ?>
$user_id has been set to the ID of the user you're grabbing the custom fields for. On an author archive template, you can get the ID like this (see http://codex.wordpress.org/Author_Templates#Setting_Up_for_Author_Information):
$curauth = $wp_query->get_queried_object();
$my_user_fields = slt_cf_all_field_values( 'user', $curauth->ID );
$my_user_fields will then be an array of the values set by fields defined by the Developer's Custom Fields plugin.
In theory you should be able to do this:
<?php $my_user_fields = slt_cf_all_field_values( 'user' ); ?>
However, looking at the code, I've done the default for the user ID wrong - it defaults using
wp_get_current_user(), which returns the currently logged in user! For the next version of the plugin, I'll make it so it tries to default to the ID of the author whose archive page is being displayed, and if not it'll default to the ID of the currently logged in user.
For now, best to get the ID yourself and pass it explicitly.