When creating Master Password, one of the primary goals was to create a password app that does not send private information over the Internet. As such, your Mac does not send your newly added sites to your phone over the Internet. Anything you create in Master Password stays on your phone (or computer) and doesn't leave it.
That said, the beauty of Master Password is that if you add, for example, twitter.com on your Mac first, and are later looking for its password on your iPhone, all you need to do is look it up on your iPhone, create it, and the password will be the same as the one on your Mac, even without sending this private information over the Internet.
To make a long answer short: your phone's new site will not automatically show up on your mac, but just add it to your mac the first time you need it and the right password will automatically show up.