Kampala has no shortage of banks and forex bureaux, but the exchange rates vary greatly between them. Generally the forex bureaux offer better rates than the banks, but there are still significant differences between the bureaux. You will also find that you are offered better rates to change cash than you get for traveler's cheques. If you take cash make it US Dollars, GB Pounds and Euros to avoid complications.
One of the best places to exchange cash and traveler's' cheques is the Metropolitan Forex Bureau where exchange rates were MUCH better than at the main banks, including Barclays. Metropolitan Forex Bureau can be found at Metropole House, 8 - 10 Entebbe Road.
If you want to use a credit card to draw cash, the only place you can do so is Barclays in Kampala. You can pay with a credit card in some larger hotels and restaurants, but only in Kampala and Entebbe.
Don't bother searching for ATM (Cash Machines) because unless you are inside Barclays Bank you almost certainly will not find one.