How one may travel to the US is determined by one's citizenship. Some citizens of other countries may travel to the US for non-immigrant purposes (i.e., travel) without a visa. Others may be eligible for ESTA, an electronic system that grants (or denies) permission to come to the US border to seek admission without having a visa.
Refugee Travel Documents are accepted by some countries as sufficient for entry, but not by all countries. The US does not accept them for visa-free travel. Without knowing your country of citizenship, we cannot say if you're eligible for ESTA. Thus, to travel to the US you must apply for and receive a US visa.
US visas for persons resident in South Africa are handled by the US Embassy and Consulates in South Africa.
Note that in applying for a visa you'll have to disclose your country of citizenship. Doing that, however, may have significant legal consequences and could imperil the validity of your Refugee Travel Document. I would not apply for a visa until after I had spoken to a solicitor or attorney who is familiar with this aspect of law.