When Mandela was released from jail, he made it clear that he was not a full supporter of communism. And this was true until his death.
However, this wasn't always the case.
In the 50's and 60's Mandela was indeed a member of the SACP (South African Communist Party). You can look this up on their website - or any South African online newspaper. Since the communists were also fighting the Apartheid government, many freedom fighters joined/allied with them in order to fight Apartheid (this of course played right into the Apartheid government's propoganda machine).
During Mandela's imprisonment, the ANC allied itself very closely with the communists in Africa (also Cuba and Russia). The MK (Mandela's military arm of the ANC) also committed many horrible crimes of terrorism during this period.
After Mandela was freed he publicly distanced himself from these actions. He also didn't continue as a member of the communists. However, his biggest challenge in uniting the country, was in keeping the different black groups/tribes working together. And so the new ruling government did not just consist of the ANC, but a coalition of the ANC, Cosatu (trade unions) and the SACP. It would have been too much of a risk if he alienated the communists, and so they became part of the government.
While Mandela was president he was the voice of moderation, keeping the communist tendencies of the ANC at bay. But sadly Mandela stepped down in 1999.
Since then the ANC has become increasingly marxist, blatantly ignoring the people's needs and arrogantly stealing from tax payers. Many people are now unfairly blaming Mandela for not foreseeing this, but in reality there wasn't much he could do about this.
Furthermore, the National Party (the Apartheid ruling party) merged with the ANC in 2005 (seriously!) and the current ANC has begun to use increasingly Apartheid-like tactics to do as they please.
Some people blame this embrace of communism on Mandela, but that isn't very fair. The truth is that he was the one keeping them in their place. I should mention that the same people that call Mandela a communist, also call De Klerk a traitor for ending the Apartheid government. They don't really mean it (since the majority of them likely supported the end of Apartheid), but with the powerlessness that most South Africans (black and white) are currently feeling, they blame anyone or anything they can about their current situation.
And while most South Africans are looking for people to blame, the current government sows seeds of discord in order to stay in power. They current ANC does not preach unity like Mandela did, rather they focus on keeping people.....apart.
Divide and conquer...