In Seller Admin > Cart Shipping Settings, the ZIP Code field there is only used for US-based sellers using USPS/UPS live rate lookups; that setting isn't used for non-US sellers at all, but unfortunately, we made that a required field, so you can enter 00000 there to satisfy the field.
We currently have no way to perform live rate lookups for non-US-based sellers, but you can still set up custom shipping calculation methods; see this help page for details:
http://www.e-junkie.com/ej/help.shipping.htm
We'd be glad to provide more specific instructions if you could explain in detail what shipping rates you want to have.
