I read all five books (you know there is no chance I am not caught up yet) but I don't remember this scene at all. Where did this happen?
From wikipedia, it happens in A Game of Thrones, Chapter 59, Catelyn IX. Robb’s host arrives at the Twins, where Catelyn negotiates their crossing of the Green Fork with Lord Walder Frey.
For allowing him crossing and his allegiance at the Twins, Lord Walder Frey extracts the price of a marriage contract, ensuring that Robb will marry a daughter of his house to be chosen after the war is finished.
Catelyn returns to Robb followed by Ser Jared Frey, Ser Hosteen Frey, Ser Danwell Frey, Walder’s bastard Ronel Rivers, and a long column of pikemen. She tells Robb that he has been granted the crossing and that all but 400 of Lord Frey’s forces will join his army. In exchange, two of Lord Walder’s grandsons will be fostered by the Starks at Winterfell, Olyvar Frey will become Robb’s squire, Arya will marry Walder’s youngest son Elmar, and Robb himself will marry his choice of Lord Walder’s daughters when the fighting is done. Robb, realizing he has no choice if he wants to cross, consents. Catelyn suggests that he should leave 400 archers and swordsmen at the Twins to ensure Lord Walder keeps faith under a reliable man. Robb suggests Ser Helman Tallhart and Catelyn agrees.
Check here for a Table of contents to help you match your book release with this chapter.
We do not see the agreement happen directly. Catelyn makes the promise for him when she negotiates with Walder Frey in Chapter 59. She discuses the terms with Robb later on in the chapter:
... "And you are to wed one of his daughters, once the fighting is done," she finished... "Do you consent?"
"Can I refuse?"
"Not if you wish to cross."
"I consent," Robb said solemnly.
Finally, at the end of the chapter we see the northern army crossing the Frey's bridge, confirming that Robb agreed to their terms.