To add an OpenStack cloud you'll need your username, password, Auth URL, your Tenant name and optionally a Region.
The first thing you should do is click " Add cloud" and select "OpenStack" from the providers' list.
Required fields here are your Username, Password, Auth URL and Tenant Name. The Auth URL might be the base url (e.g. https://192.168.1.101:5000) or a full path (e.g.https://192.168.1.101:5000/v2.0/)
Optional field is the Region name (if you've specified another Region than the default, which is RegionOne).
Once you click on Add, Mist.io will try to authenticate with OpenStack. If all goes well, you should see a list of your VMs in the Machines section.