4
Is this the only config option you can't change? If so, read on; if not, it is probably something else.
Did you install with 'fantastico'? If so, search the forums here for 'fantastico' and you'll find there is a bit of an issue with this installer and a few versions of xoops. If you did use fantastico, you may want to consider the possibility of installing XOOPS yourself via the XOOPS installer (if possible with your hoster). Otherwise you may need to get your hands dirty with a little bit of MySQL to fix a couple of errors made by fantastico.
[EDIT: I've just added a FAQ for the Fantastico issue on the wiki as it has come up several times now.]