You will pick up the first part of the Muspelheim seed fairly early in the game. This will trigger a quest called the Crucible where you will be tasked with finding the second seed piece to allow you to get access to the realm through the gateways. Here is where to find the second piece.
You will need to progress a decent amount through the main story before you can get this item. Without giving away spoilers, you will need to have reached a point in the story where you have the ability to destroy the orange glowing cracks in stone walls. This item is unlocked when you turn a certain item belonging to Odin into something else by going underwater. That’s about as spoiler free as I can make it.
Once you have this item to destroy the orange cracks in the wall, you will need to make your way back to Freyer’s camp. You will probably have noticed a golden chest in Freyer’s camp before. This is the chest that contains the second Muspelheim seed piece.
You will find some stones to hop across the river and you will reach the wall with the orange cracks in it. Implant the spear into this and detonate it to blow up the stone wall and you will be able to go over to the quest and get the second seed piece to complete it. The quest will update and you can now travel to Muspelheim.
If you are unsure how to find the gold chest in Freyer’s camp that contains the second Muspelheim seed fragment, the video guide below will show you exactly where to go and how to find the chest.