When mapping your model to an underlying implementation, architects use connections to runtime engines such as SOA Suite and Service Bus. The connections are listed in the mapping user interface. These connections are created and configured by administrators. If a connection has not been completely configured or tested, it is possible that architects will not be able to browse the runtime engine associated with that connection. Work with your administrator to validate that connections are configured properly and have been tested.
Another reason might be that the runtime engine associated with a connection is offline. For example, if a SOA Suite server has been taken offline for maintenance, an architect will not be able to browse that server and map milestones until the server is brought back online.