X-cart doesn't have a "homepage.tpl". If you want your marquee to be directly below "Welcome to..." or "Welcome back..." then either place your code, or insert an include (probably a better idea) in the area you have commented out ( {*{$lng.txt_welcome}*} ).
Also, there appears to be some formatting errors in your welcome.tpl. You have "open paragraph (<p>) statements without "close paragraph"statements, and 1 of these is within a "if" statement.
Code:
{if ($active_modules.Greet_Visitor ne "") and ($smarty.cookies.GreetingCookie ne "") and $logout_user eq ''}
<H3>{$lng.lbl_welcome_back}, {$smarty.cookies.GreetingCookie|replace:"\'":"'"} </H3>
{else}
<H3>{$lng.lbl_welcome_to} { $config.Company.company_name }</H3>
{/if}
<p>{*{$lng.txt_welcome}*}</p> <<< INSERT YOUR MARQUEE HERE
<p>{include file="customer/main/homepage.tpl"} </p> <<< WHAT IS THIS???
{if $active_modules.Bestsellers ne "" and $config.Modules.bestsellers_menu ne "Y"}
{include file="modules/Bestsellers/bestsellers.tpl"}
{/if}
{include file="customer/main/featured.tpl" f_products=$f_products}