You actually own a private key, which generates the public address which is given to you.
For example, let's say your public address is:
The private key which generates that address is:
5HtxkoHrU6iGJXaNCQ5iAZBEXgTWk13k14SFpQrNJHVrJuTxN78 (Do not use this private key!)
To be able to add that address to your wallet, you need to add your private key to your wallet. What your wallet basically does is to show the balance of all your private keys. You can send and receive BTC through these addresses.
So right now you have BTC on the private key (or public address) which you ordered on Virwox. If you want to see it on your wallet, you can either add the private key to your wallet, or send those BTCs to the address generated by the Multibit wallet.
BTW: If you're going to send the BTC, please make sure that you backup your wallet just in case.