In order to set up the subscription go to the settings menu and press Subscription Purchase / Restore.
On this page there is a link to the MedGasLog Cloud Service that appears at the top of the page. Click on this link:
It will ask you for your Apple iTunes ID.
There are a few more screens that are required by apple including sharing some basic information. All of these are required in order to complete the subscription.
Once this is done there should be confirmation. To check, if you save a case it should come up with no message under the save text (i.e. previously it would have told you how long left you have of your free trial).
If you want to stop your subscription then please look at this article.