In Configuration > My System go to the 'Subscriptions' tab:
This tab allows you to view various pieces of information regarding the performance of your Opsview Monitor software, including
Number of hosts / services / collector servers
Software key expiry date
Expiry date of each feature (if you have purchased the Network Analyzer module separately, etc)
Aside from displaying information about the active software key, you can also add a new software key via this page by simply clicking on 'Add a Software Key', which will load the standard activation page as below:
To manually activate a new Software Key or for more information please navigate to your 'https://[YourServer/activation/manual' and follow the steps.