At the end of Moshe's life he gathered all the Jews together for a last big covenant with G-d (Nitzavim). This gathering was for everyone, including "the ones that are not here today" (Devorim 29, 14). This means even the unborn future generations that are bound to the covenant, just like the branches are bound to the tree. Alternatively, Moshe was talking even to the unborn children, because all the unborn souls were standing there.
And so does the Talmud say in Shvuous 39a: how do you know that Moshe swore in even future generations and future converts? Because it says "even these that are not here". And we only know the mitzvos that they accepted at Mt. Sinai, but how do you know that the covenant applied even to future mitzvos like the reading of the Miggila? Because it says "Keemu Vekiblu". They were mekabel (Purim) what they were already mekabel before (By the covenant). (Shvuous ibid)
And don't ask stupid questions, like "how can you be mekabel something that didn't happen yet?" Because the Torah is the blueprint of the world and it comes before the world. So the only thing that didn't happen yet was the "playing out" of the physical story of Purim, in this world. But the 'essence' of the story was in the Torah long before that. It's like G-d showing Moshe a real life movie of Rabbi Akiva teaching Torah to his disciples (Munachos 29b), even though Rabbi Akiva was not until 1,000 years later. (Yes, if G-d wanted he could have shown Moshe the gas chambers)
And if you have a question "how does this fit in with free choice?" "Good morning! You just woke up". Is this the only thing bothering you? By us we say a simple thing. It's called "I JUST DON'T UNDERSTAND" and life goes on. But by the Modern Orthodox if you don't understand something you run to god #2 (The colleges), and you start saying crazy things, like "since we don't find in secular history that all the waters of the world split (By kreeyas yam suf), this is proof that it never happened, and the Medrish is not for real. (Dovbear)
Now the Talmud also uses our verse (Devorim 29, 14) to teach that the future converts (Mazel) stood at Mt. Sinai. I guess if the Mazel was at the covenant then certainly it stood at Mt. Sinai, where there was the marriage of G-d and the Jews, and G-d spoke to us personally. And if the converts Mazel stood at Mt. Sinai then certainly the future Jewish souls were there, like the Talmud groups them together in Shvuous.
The bottom line is that we were all there at Mt. Sinai, and G-d spoke to us personally (The first two). So if you wanna use your free choice to go OTD that's fine, but don't mix G-d into this. Like all of a sudden you're a certified Atheist. You hear this! The guy wants to 'shtuup' around without any guilt, so all of a sudden he's an Atheist. Let's see what happens when his "thing" stops working. (See Apr 4, 2013: Dec 12, 2012)
P.S. I just saw today's Dovbear and I would like to ask him a few questions. #1, "How did Darwin make that your "hole" is exactly in the middle?" Why isn't it like pushed off to the side a little? #2, how did Darwin make your penis and your balls exactly in the middle? why doesn't your penis come out of your left ear? #3, how did Darwin make that you can tell the difference between a fart and diarrhea, and you don't have daily accidents until you become an 'alter kocker'? #4, are you really a Catholic Priest? Were you at Mt. Sinai?