For me the issue resolved only when I bypassed my proxy and connected to the Internet directly. When the proxy is enabled, the 'Add' button never appears. When it's disabled, the 'Add' button appears as expected. This is not an issue when adding a Twitter account, so apparently the Facebook account code doesn't traverse the proxy as gracefully as the Twitter code.
For me the issue resolved only when I bypassed my proxy and connected to the Internet directly. When the proxy is enabled, the 'Add' button never appears. When it's disabled, the 'Add' button appears as expected. This is not an issue when adding a Twitter account, so apparently the Facebook account code doesn't traverse the proxy as gracefully as the Twitter code.