I'm going to say going from here that it is 1961, as it seems to be when they start engaging in terror practices although I doubt the list goes back that far. Yet from the graph of incidences I see the first one recorded here is on 1976-11-30.
I'd need to coordinate these with when the list really started though.
So I did some more checking and really the ANC was first on the Terrorism Watch List, which looks to he the predecessor of the FTO list prior to the Patriot Act. It looks like at first, according to USA Today (I know, but bear with me) that the ANC was considered a Terror Organization by the former Apartheid government of South Africa. Which fits in with the timeline I noted above, where the government the ANC was seeking relief from considered the ANC a terror organization. Considering the time that would probably be true. Now trying to tie down the time period I get two different era's so I am not sure what makes more sense here.
Either it was during the Reagan administration:
In the United States, the Reagan Administration insisted on listing
the ANC among terrorist organizations. Conservatives often exhibit a
tragically limited time horizon. One consequence of this was that long
after his release from prison in 1990, Nelson Mandela still found
himself flagged on the U.S. terrorism watch list in 2008, which
required him to obtain special permission to enter the USA.
or if you go by Senator John Kerry, who helped support the amendment to remove the ANC from the FTO then it's back to the 1970's.
In the 1970s, the U.S. government placed the members and
representatives of the ANC, including its leaders like Mandela, Walter
Sisulu, and Govan Mbeki - the father of South African President Thabo
Mbeki - on the U.S. Terrorist Watch List.