look astro faggot i just posted the fucking relevant passage. We know you have never read the books. read the quote I posted from A Storm of Swords. It's the last Catelyn chapter before they arrive at the Twins.
Here's the last bit of it, later on in the chapter. Where the "letter" bit comes from:
Catelyn turned to her son. "I mean no offense to Lord Jason," she said stiffly, "but if I cannot continue on with you, I would sooner return to Riverrun."
"I left my wife at Riverrun. I want my mother elsewhere. If you keep all your treasures in one purse, you only make it easier for those who would rob you. After the wedding, you shall go to Seagard, that is my royal command." Robb stood, and as quick as that, her fate was settled. He picked up a sheet of parchment. "One more matter. Lord Balon has left chaos in his wake, we hope. I would not do the same. Yet I have no son as yet, my brothers Bran and Rickon are dead, and my sister is wed to a Lannister. I've thought long and hard about who might follow me. I command you now as my true and loyal lords to fix your seals to this document as witnesses to my decision."
A king indeed, Catelyn thought, defeated. She could only hope that the trap he'd planned for Moat Cailin worked as well as the one in which he'd just caught her.
If you had ever read this chapter you would know what the fuck we are talking about. Here let me break it down for you:
1) Robb & Co are trying to return to the North and reclaim it from the Ironborn, they have to go to the wedding at the Twins to settle the issues in the Riverlands first, which as you know from watching the show turns out to the be the Red Wedding.
2) They had recently gotten word about Theon's betrayal and the alleged death's of Bran and Rickon, as well as Tyrions wedding to Sansa. Arya's also presumed dead.
3) They also had gotten word of Balon's death and Euron's return to Pyke claiming the Seastone chair, meaning that all of the prominent Ironborn currently holding territories in the North are forced to return to Pyke, meaning that only a small number of ironborn will remain holding Deepwood Motte, Moat Caitlin, etc. Now is the time to retake the north from the Ironborn.
4) Except Moat Caitlin cannot be taken from the South. Initially Robb wants to go through the Vale of Arryn and sail to the north and then march south on Moat Caitlin, but Lysa will not open the bloody gate to his army.
5) So the new plan is to travel through the Neck with crannogmen guides provided by Howland Reed. The great jon will lead an attack from the south along the causeway, with Roose Bolton leading a 2nd army. Robb's forces will sneak through the neck and attack from the west, with the help of the guides from Reed.
This is the part in question where "robb's will" may or may not have been sent. Robb sends Galbart Glover and lady Maege Mormont into the neck to find Howland reed, to get the guides for this assault. They are carrying other letters, false orders, should they be captured by the iron born while en route. It's possible that they also have his Will with him, there's no way to know.