Sadly you can't transfer Pokemon from Black and White to X and Y just yet. Some time in late December, two apps will be added to the eshop. One being Pokemon Bank, and the other being Poke Transfer.
Pokemon Bank allows you to move Pokemon from Black and White to X and Y freely and at anytime. It also allows you to store your Pokemon as well. Pokemon Bank will have 100 box's of storage (3000 Pokemon) and will be a paid service.
Poke Transporter than will be free and will only do transfers to Black and White to X and Y without the storage.
Pokemon Bank and Poke Transporter also make sure the Pokemon you are moving are legitimate as any generated Pokemon will not be allowed to be transferred through these services.
When they say "Generated" they mean Pokemon that you used an Action Reply code to make appear in your box or party. I don't believe this applies to Pokemon you've "caught" with a masterball while using the Pokemon Modifier code but this is unconfirmed.