unfortunately the changes between the plugin and and 1.5.6 are extensive, and my understanding of zencart does not cover a lot of the new functions such as the use of notifiers etc. In addition a number of the functions (includes/functions/functions_customers) seems to be based on 1.5.1 core and the changes to the new 1.5.6 are extensive.

your_admin/customers.php for instance seems to be fully incorporated, with other files functionality moving completely - Your_Admin/includes/functions/functions_customers.php has now been moved to the catalogue side (according to the file)

I think I'll wait for someone more experienced to fix the changes for 1.5.6, although I'm quite willing to donate to the coffee/beer fund to help implement this