Those look like spanner screws. They are one of the common types of security screws, you should be able to get a screwdriver without too much trouble, on-line or in a hardware store (there are several sizes though, so be careful to buy the right one). Alternatively
iFixit sells a huge kit of security bits for $25, but that's only useful if you encounter security screws more or less regularly.
edit: They are apparently also known as snake eyes screws, or even as pig nose screws.