You can link your current Xbox Live Gamertag and PlayStation Network ID to your Ubisoft Account using the ctOS companion app. In the bottom left hand corner, you can find the Logged Networks area. This will show you which accounts you're currently logged in with on the app.
When the Playstation Network and Xbox Live icons are grayed out, this means that you're not currently logged in to either network. Clicking on either respective icon will allow you to log into the given network.
Once you've logged into your PlayStation Network and Xbox Live account, the logged networks will become colored.
If you do not have either a free Xbox Live or free PlayStation Network account, you can create one at the following links: