Check your mail box. Use the activation link in their email to activate the offer.
T&Cs from the email,
Valid only for members that receive this bonus invitation email from Cashrewards. $5 bonus must be activated prior to making an eligible purchase of AUD $50 (inc GST) or more through Cashrewards. Valid for single purchase only. Purchase must be made between 05:00pm AEDT 28/01/2020 and 11:59pm AEDT 31/01/2020. Bonus will not be added to your Cashrewards account at time of purchase. It will be credited separately as approved by 10/04/2020. Transaction is valid if made online via desktop browser, Cashrewards mobile app, Cashrewards Notifier, mobile browser or in-store at participating stores via a linked Visa / Mastercard. See full terms.
Got it. Thanks op!