Subscribing to a service offering is very straightforward.
Go to the "Service Offerings" menu and click on the Service Offering you want to subscribe to.
In the new screen that has opened, click on the "Subscriptions". This opens a view with a button "Subscribe". Click on that button and you see a modal where you can select an Account from your Company to associate with the subscription. You need to approve the Terms of Service before allowed to confirm the subscription.
If your account is already subscribed to the service offering then it will not show up in the drop down menu
If your Company has a restricted account and the user doing the subscription doesn't have the proper entitlements, then the restricted account is not visible in the drop down.