How to show alert pop-up in in Cocoa on macOS?

You can use NSAlert in cocoa. This is same as UIAlertView in ios. you can pop-up alert by this

NSAlert *alert = [NSAlert alertWithMessageText:@"Alert" defaultButton:@"Ok" alternateButton:@"Cancel" otherButton:nil informativeTextWithFormat:@"Alert pop up displayed"];
[alert runModal];

EDIT:

This is the latest used method as above method is deprecated now.

NSAlert *alert = [[NSAlert alloc] init];
[alert setMessageText:@"Message text."];
[alert setInformativeText:@"Informative text."];
[alert addButtonWithTitle:@"Cancel"];
[alert addButtonWithTitle:@"Ok"];
[alert runModal];

Swift 3.0

let alert = NSAlert.init()
alert.messageText = "Hello world"
alert.informativeText = "Information text"
alert.addButton(withTitle: "OK")
alert.addButton(withTitle: "Cancel")
alert.runModal()