I think the problem may have something to do with your installation; the following worked OK for me:
14:15:12 SQL> create table serie(x smallint, y smallint, z smallint);
Table created.
14:15:28 SQL> insert into serie values(1,2,3);
1 row created.
14:15:37 SQL> insert into serie values(4,5,6);
1 row created.
14:15:45 SQL> insert into serie values(7,8,9);
1 row created.
14:15:53 SQL> commit;
Commit complete.
14:15:55 SQL> select xmlgen.getXML('select * from SERIE') from dual;
clsXMLGEN.GETXML('SELECT*FROMSERIE')
--------------------------------------------------------------------------------
<?xml version = '1.0'?>
<ROWSET>
<ROW num="1">
<X>1</X>
<Y>2</Y>
<Z>3</Z>
</ROW>
<ROW num="2">
<X>4</X>
<Y>5</Y>
<Z>6</Z>
</ROW>
<ROW num="3">
<X>7</X>
<Y>8</Y>
<Z>9</Z>
</ROW>
</ROWSET>