You should avoid manually assigning AppInsight to a node manually. Much of the necessary information required for monitoring is pre-populated when discovered through "List Resources" for WMI or SNMP managed nodes. If the node is managed via SNMP and added through "List Resources" you will be prompted to provide SQL credentials. The fact that you weren't prompted for credentials tells me that either AppInsight for SQL was assigned to the node manually, or the node is managed via WMI.
If the node is managed via WMI additional information is polled for the application, such as Processes/Services, Disk I/O, Windows Event Logs, etc. If the node is not managed via WMI then this information is collected and should not appear on the AppInsight for SQL or Database details views.
My recommendation is to manage the node via WMI, selecting the SQL instances to monitor through the Add Node Wizard. If your Windows Credentials used to manage the node via WMI don't have adequate permissions to connection to the SQL server, you will need to edit the assigned application and provide SQL credentials for the rest of the application monitoring. Normally these credentials would be the "sa" account or another account with equivalent permissions. If you're not comfortable using the "sa" account you can create a non-admin level account with read-only permissions to the SQL server following the steps outlined in the KB article below.