It may be down to the browser version you are using. Older browsers don't always work correctly on modern sites as they were built before newer website technologies existed. Try using an alternative browser such as Google Chrome or Mozilla Firefox.
Should you still be unable to add your address then try adding your address manually by clicking the "I cannot find my address" link and filling out your address manually. Be sure to enter your details correctly so that we can deliver the order to the correct address.
If you are still experiencing problems after trying the above steps then please contact us and we will be happy to assist you further.