Hard to diagnose on the basis of a post on the www, but, as a generic, you must eliminate all possible reasons for non-feeding as inferred by others here as well.
That is parasites, ill health, etc.
Ensure cage is best for snake, (appropriate heat gradient, clean water, good hide, etc).
Assuming snake not on death's door, allow time to settle in once all adverse factors eliminated.
After all above done, try various foods and methods.
If all fail, then force/assist feed.
Any/all first feeds should be relatively small and don't re-feed until food out of stomach. Repeat feeding process and gradually increase food size.
Uusually snakes new to captivity will self feed and those that don't (see above) usually will after the first or second force/assist feed.
Mambas are a large generalist species so you shouldn't have problems.
Happy herping.