If you are unable to access the Promethean App Store from your ActivConnect, first check that you are connected to the Internet, either via Ethernet or Wi-Fi.
If you can access the Internet, you might need to ask your IT administrator to whitelist the URL of the Promethean App Store.
This is http://promethean-store.store.aptoide.com/. Please note that visiting this site from a web browser displays two search bars. The one at the top of the page will allow you to search the Aptoide store, and will not filter or restrict content. The Search application... search bar will allow you to search safely for apps on the Promethean Store.