Hi Mandy,
The easiest thing to do is bring South African rand, as they are legal tender in Namibia. One rand = one Namibian dollar. You will get your change in either rand or dollars but make sure you get rid of any Namibian currency before you leave. One final thing - make sure your rand is the latest issue (with Mandela's face on them) rather than the previous ones, which had animals, as these are no longer accepted.
I did this tour in May last year and had a fantastic time - I am sure you will too!
Best wishes,
Bob