Crib assembly. Ok, I bought this Cosco 3in1 crib at a baby store that sells used products. I know

Question

Ok, I bought this Cosco 3in1 crib at a baby store that sells used products. I know it has all the pieces since it was put together before I bought it. HOWEVER, I cannot figure out how to get it together correctly. I have it all right except the side that drops down keeps falling off so I know something isn't right. At the store when it was assembled, I was able to move it up and down without it falling on me. The model number is: wm1633. I've looked all over the internet and I can't find it anywhere! Please help!