How do I unlock this electrical meter cabinet?
You buy a utility-key (AKA services cabinet key) or poke around with needle-nosed pliers



The X shaped metal tool (four-way utility-key) has four different key shapes commonly used. It also has a useful plastic adapter for hex drive and a two-ended flat/phillips bit.

The keys shown in my photo work on both gas and electric meter boxes in the UK, you can buy them in most hardware/DIY shops.