I use windows not a mac so I may be off base here. I often have problems getting pallets to doc where I want too. I find I need to jiggle the palette about into where I want to dock past the boundry. Photoshop then seems to get the idea and will turn the boundary blue. When blue I can let the mouse button up and it will dock to the boundary. Here the tool palette had to make it wide to be able to drag it.