To add a Linode cloud, you'll need a valid API key. To get it, login to your account in Linode and select 'my profile'. After you provide your password that is asked for confirmation, click on "Api Keys" and create a new API key.
To use Linode with mist.io, login to https://mist.io and click on the 'Add cloud' button at the bottom right
Select Linode from the provider’s list and paste your Linode API key.
Once you click on Add, Mist.io will try to authenticate with Linode. If all goes well, you should see your VMs listed in the Machines section.