I might not fully understand your question but it seems that $html->find
simply returns an array, in your case an array of tables:
$tables = $html->find('table.wikitable');
You can then loop through your tables and find the td's in each table:
foreach( $tables as $table )
{
$tds = $table->find('td');
foreach( $tds as $td )
{
...
}
}
If you only want to target the second table you can use:
$table = $tables[1];
Or something like that.