There are two main categories of scripts in Anime Studio, tool and menu scripts, and each one must be installed in a slightly different way to work with Moho.
Menu scripts are easy - inside the Anime Studio program folder is a sub-folder called "scripts". Inside this is another folder called "menu". If you place a menu script in that folder, it will automatically appear in the Scripts menu when you start up Moho. This folder is further subdivided into different categories that appear as sub-menus in Anime Studio, and you are welcome to install your menu script in whichever category makes the most sense.
Tool scripts are a little more complicated. Look at the "scripts" folder in the Anime Studio program folder again. Notice the "tool" sub-folder. All tool scripts must be placed in this folder. If you create a new tool script and put it in that folder, it will appear at the bottom of the Anime Studio toolbar in a new tool category labeled "Other". This is the simplest way to install a new tool script. However, if you want to control where exactly your new tool should appear, look at the file called "_tool_list.txt" in the same folder. This is a text file that you can edit with a basic text editor like Notepad. In this file, you'll see that tools are organized into groups, and arranged in a logical order. You are welcome to modify your copy of this file to install your own tool scripts wherever on the toolbar you wish.