Some general advice:
With many DAB radios you can do the same job as 'retuning' a TV simply by pressing the 'Scan' button. That then stores all available channels, which can be accessed by pressing the Up/Down ''Tune' buttons. Storing a channel into memory is often achieved by holding down the relevant memory button for a few seconds.
That might help you but, if not, you need to get hold of a copy of the instructions (or speak to someone who has them in front of them). 'Bush', as a manufacturer, no longer actually exists. Argos bought the brand name and applies it to goods sourced from several different companies. So it's Argos that you need to contact. They've got a telephone support line for Bush products on 0845 604 01 05. If you give them a call they'll either be able to tell you what to do or email you a copy of the instructions.