Having trouble setting up iCloud Keychain in multiple devices

I set up for using iCloud Keychain on my iMac but am having problem getting through approval process for iPad. iPad demand either use of Security Code which I do not have or approve it from another devicer that uses iCloud Keychain. iCloud Keychain on iMac's preference says "waiting for approval from other device. clicking Detail tells me "Approval request has been sent to the other device using iCloud Keychain". Yet I do not see the request on my iPad. I am at complete loss. Appreciate any hint to get out of this mess.