1. You can't go to Freedom Isles until you win a boss fight. You'll know when you've achieved this. It's part of the main quest, so just do what the king asks.
2. Find some Swamp Snakes for Sleem in Marshan Swamp. You may have to run around some hard enemies, but you should be able to find some for sale. You need to buy them into your army (about 25 from memory) and then talk to him so that he can eat them.
