You'll have to use your spellbook to select the metamagic, the radial menu doesn't work anymore for these, it's a known bug that was reported as soon as it happened.
It's unfortunate for the beguiler amulet and the master warmage staff as otherwise you could cast using those slots while below the caster level required for 8th & 9th spells, and if you still had it slotted on a quickbar from way back to just before we had the NWN version update, you still can.