Open the chromecast app
- Wait for the list of devices to populate.
- Select the relevant device.
- Wait for the "Connecting to XXXX" dialogue to finish.
- When all the device info etc pops up, press the menu overflow button.
- Select "Check for offers" and follow the instructions. (You must agree to send your serial number).
- It should then allow you to open the play store and redeem the voucher immediately.
In both cases you must be connected to the same Wifi network as the relevant chromecast and on a PC/laptop, you need to be using the Chrome browser and have the Google Cast extension installed.