Quote:
"Identifier IDD_DIALOG1 is undefined"
This is letting you know that there is no resource with that ID. It's likely that you changed the dialog resource name and didn't change it in the code where that was originally inserted for you.
In the resource editor, what's the label (it's likely to have the same IDD_ prefix) associated with your dialog? That label needs to appear everywhere where the original IDD_DIALOG1 label appears (in the code, do a quick search within your solution to figure out where you left the old identifier behind).