You will need the services of a 'transitair' once you get there, in my opinion just semi-official bribary. This person takes your money and then pays all the taxes and customs officials and ofcourse keeps something for himself.
When I was in Dakar the customs wouldn't deal with me directly, they all said: 'go to the transitair', and I heard from the embassies that they also use them.
The reason they don't want to deal with you directly is ofcourse that via the transitair they get something extra. The transitair wanted 300 euro per motorbike, I heard later we should have only paid 140 euro (accually we didn't pay at all but thats a different story).
I hope you get around this, anyway it is going to cost some money as they are very good extracting it from the 'toebabs'.