The dialog from the screenshot looks like a Confirm dialog and probably this is the reason for the failures you encountered when using Alert dialog handler. This said, can you try the following code as it should be the same as yours, but using a Confirm dialog:
// Add a confirm dialog to monitor
ConfirmDialog altDlg =
ConfirmDialog(ActiveBrowser , DialogButton.OK);
// Given that there were not dialog attribute set, the manager will not start the monitoring.
// You need to invoke the monitoring
// Then invoke the action that triggers the dialog
Additionally, please note that, you need to explicitly start the dialogMonitor and trigger the dialog appearance after the dialog is added. In other words - the consequence of the actions in the code above need to be explicitly followed.
I hope this works for you. In case you need further assistance, please let me know.