All blockchain transactions are based on private-public key pairs, so that other chain members can verify transactions, but only the appropriate party can create them. I recommend reading about this subject online, since it's true of all blockchains in general.
A wallet address is derived from a public key using a one-way hash function and acts as an identifier on the chain. You can also research this online in the context of bitcoin – it also applies to MultiChain.
You shouldn't try reading wallet.dat directly – instead, use various APIs to extract private/public keys and addresses.
That looks like the correct blockchain directory for Windows. The chain itself is stored in various files within that directory.