To enable Cloud Build with Unity Collaborate, first open the Collab History window by clicking History from the Collab toolbar, or Window > Collab History.
In the Collab History window, click configure now.
This will launch the Cloud Build website, where you can select which platforms you wish to build for. See the Cloud documentation for more information.
Is something described here not working as you expect it to? It might be a Known Issue. Please check with the Issue Tracker at issuetracker.unity3d.com.