Thanx paladino
...but WHERE do I update the mace?

The ilithium is obtained from ***** ******** in the C.Coronet. You need to get the sculptors quest from the sculptor in the Temple District (Helm?), possibly only available after the lidless eye quest. Talk to the ore merchant in Waukeens once the quest is obtained. The route you take is up to you. But I believe there is only 1 piece of ilithium available. Once obtained talk to Cromwell.

Hope this helps.

I know that it's possible to update the Mace of Disruption using the illithium ore, and also that you can update the silver Horn of Valhalla using a Beljuril gem (you can update it twice I believe but does the second time require a Beljuril as well? and if not, then what kind of gem?).
However, I have no idea how to do it!! I assume you need to go to some merchant or so, because Cromwell can't do it!
Who can help me with this? I've tried everything and I'm really dying for a solution!

To update the horn, try the merchant in Waukeen's Promenade, over by the art dealer. I've never had him work on one of my horns, but if you enter the right dialogue script, he says that he works on horns.

I've never found illithium ore, but I know it can be done.

Thanks!
Does anyone know about the Mace, or any specifics on which gems are to be used with the Horn of Valhalla?

you need the beljuril and a diamond, and the mace becomes a +2, +3 vs. undead instead of +1, +3 vs. undead