You just need to import the Distribution certificate (.p12 public+private key pair or create certificate request if you only have .cer public key file). Note that you can have as many certificates as you wish in Icenium.
To publish with Mist check out new publish wizard guide from documentation.
You may have to create an App Store distribution provision and import it to Icenium when prompted.
Finally to upload to App Store (right now possible via Icenium Mist) you first need to setup your application details in iTunes Connect and make the application binary Waiting for Upload.
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