One of the definitions of antithesis (in literary terms) is some passage in which opposite, or nearly opposite, ideas are both presented as truth.
So, in antithesis, the author gives us two ideas and says they are both true even though we know they can't really be.
So where in the poem does this happen? I'd say the whole second stanza shows antithesis. Let's look at it:
Then took the other, as just as fair,
And having perhaps the better claim
Because it was grassy and wanted wear,
Though as for that the passing there
Had worn them really about the same
Here, Frost is saying first that one path was better because it was less worn. Then he says that the people passing had worn them both equally. This fits the definition of antithesis.