You need to change the $template_sys_menu in your theme.php to
$template_sub_menu = <<<EOT
<table border="0" cellpadding="0" cellspacing="0" align="center">
<tr>
<!-- BEGIN custom_link -->
<!-- END custom_link -->
<td class="top_menu_left_bttn">
<a href="{ALB_LIST_TGT}" title="{ALB_LIST_TITLE}">{ALB_LIST_LNK}</a>
</td>
<td><img src="themes/igames/images/menu_spacer.gif" width="2" height="35" border="0" alt="" /><br /></td>
<td class="top_menu_bttn">
<a href="javascript:;" onmouseover="MM_showHideLayers('SYS_MENU','','show')">@</a>
</td>
<td><img src="themes/igames/images/menu_spacer.gif" width="2" height="35" border="0" alt="" /><br /></td>
<td class="top_menu_bttn">
<a href="{LASTUP_TGT}" title="{LASTUP_LNK}">{LASTUP_LNK}</a>
</td>
<td><img src="themes/igames/images/menu_spacer.gif" width="2" height="35" border="0" alt="" /><br /></td>
<td class="top_menu_bttn">
<a href="{LASTCOM_TGT}" onmouseover="MM_showHideLayers('SYS_MENU','','hide')" title="{LASTCOM_LNK}">{LASTCOM_LNK}</a>
</td>
<td><img src="themes/igames/images/menu_spacer.gif" width="2" height="35" border="0" alt="" /><br /></td>
<td class="top_menu_bttn">
<a href="{TOPN_TGT}" onmouseover="MM_showHideLayers('SYS_MENU','','hide')" title="{TOPN_LNK}">{TOPN_LNK}</a>
</td>
<td><img src="themes/igames/images/menu_spacer.gif" width="2" height="35" border="0" alt="" /><br /></td>
<td class="top_menu_bttn">
<a href="{TOPRATED_TGT}" onmouseover="MM_showHideLayers('SYS_MENU','','hide')" title="{TOPRATED_LNK}">{TOPRATED_LNK}</a>
</td>
<td><img src="themes/igames/images/menu_spacer.gif" width="2" height="35" border="0" alt="" /><br /></td>
<td class="top_menu_bttn">
<a href="{FAV_TGT}" onmouseover="MM_showHideLayers('SYS_MENU','','hide')" title="{FAV_LNK}">{FAV_LNK}</a>
</td>
<td><img src="themes/igames/images/menu_spacer.gif" width="2" height="35" border="0" alt="" /><br /></td>
<td class="top_menu_right_bttn">
<a href="{SEARCH_TGT}" onmouseover="MM_showHideLayers('SYS_MENU','','hide')" title="{SEARCH_LNK}">{SEARCH_LNK}</a>
</td>
</tr>
</table>
EOT;
It's in a conditional, so modify the first if you use a custom link, the second if you don't.
Then in style.css file set the #SYS_MENU to text-align: center and adjust the left position until it looks correct.