How can you get weapons at the start of Metal Gear Solid 4

I remember having trouble with the same thing. After completing the game watch the credits and go back to the main menu. Press X on the load option and select your Completed Game save. It will pop up a screen asking you what difficulty you want. It doesn't matter what you choose but you will start at the beginning of the game again. When you meet up with the Mk: II you will receive the Operator, Tranquillizer Gun, the, Solid Eye and finally the Mk: II along with you other weapons, items, face camo, and costumes. Good Luck