If it's the plate with the bolt with the three holes in it then I think you need to go to a dealer with the chassis number, assuming the alloys are OEM or dealer-fit. The tool to open it is a sort of T-bar which fits into the top of it.
I know this because I bought a spare for mine in the UK as I wasn't comfortable only having one.
I had this problem recently and contacted the dealer directly. They wanted to charge me €40 for this key which I argued should have been included with the car. They eventually gave it to me for nothing. If you bought it new from the dealer you should contact them directly for it.