[cflsystems]

Try this, replace "in_stock" with the name of the field you created. This goes in skin1/customer/main/product_details.tpl for 4.3

{foreach from=$extra_fields item=v}
 {if $v.active eq "Y" && $v.name eq "in_stock"}
  {if $v.field_value eq "0"}
   {* no items in stock *}
   SHOW MESSAGE FOR ITEM OUT OF STOCK
  {elseif $v.field_value eq "-1"}
   {* item discontinued *}
   SHOW MESSAGE FOR ITEM DISCONTINUED
  {else}
   {* any value (or no value, empty field), except 0 and -1 *}
   SHOW MESSAGE ITEM IN STOCK
  {/if}
 {/if}
{/foreach}