how to activate and necklas of magic missile ? or one with schield spell?

Have you got the "Use Inventory Item" button in the toolbar, the one that looks like a pack?

If not, right click on the toolbar to add it.

The "Use Item" button is not available by default. You will have to replace one of the existing shortkey buttons with this. To replace buttons, right-click the button you wish to replace and you will get a collecion of available buttons.