From a quick search, some of the results that I've found seem to imply that Lebanese addresses can have postal codes, which consist of 8 numbers - the first 4 for the region, the second 4 for the building. Are you being forced to enter a post code ?
What sort of visa card are you trying to add, debit or credit ? If it's a debit card then I don't think that they are still accepted as a valid payment method - they are not listed on this page and there have been a number of posts recently about them being declined
If it's a credit card then is it registered to exactly the same name and address (including format and spacing etc) that you have on your iTunes account, it was issued by a bank in your country and you are currently in that country ? If it is then you could check with the card issuer to see if it's them that are declining it, and if not then try contacting iTunes support and see if they know why it's being declined : http://www.apple.com/support/itunes/contact/ - click on Contact iTunes Store Support on the right-hand side of the page, then Account Management
