Sorry, I didn't even turn my computer on last night but, finally, here's a generic step-by-step for installing a script in Moho:
1. In Moho, look under the File menu and select Open Custom Content Folder. A folder should open. This is where you copy scripts and other custom assets to Moho. For example, if you copy a script that modifies an existing tool, Moho uses this one. This way you won't destroy the original script found in the main program folder, just in case you need to switch back to it.
2. Open the Scripts folder and keep this folder open because you will be dragging files to it.
2a. If the Open Custom Content Folder is disabled, that probably means you didn't set this folder when you first launched Moho. No worries; just open the Preferences window from Edit and set your Custom Content Folder from there. The folder can be almost anywhere you keep your documents, like the Documents folder or on a secondary drive. If you choose a secondary drive, it should be one that's always accessible to Moho.
3. Quit Moho but keep the Custom Content folder window open.
4. Download the script and unpack the file(s). Usually, the contents will have a file structure starting with a folder called 'Menu' or 'Tool'. If it only contains a .lua file, you need to know what kind of script this is. If you only see the .lua file, it's probably a 'Menu' script. If there's a .lua file and a .png file, then it's probably a 'Tool' script. (In which case the .png is a tool panel icon.)
5. Now copy the files to their appropriate locations in the Custom Content folder's Scripts folder.
If you have a Menu folder, you can drag it to replace the Custom Content folder's Menu folder or drag the contents
of the Menu folder to the appropriate category folder inside. Theses categories represent the categories listed under the Scripts Menu in Moho. (You can even add your own categories here.)
If you have a Tool folder, you can drag it to replace the Custom Content folder's Tool folder or drag the contents
to the Custom Content folder's Tool folder. Inside the Tools folder, you'll notice that there are no categories here, the .lua and .png files all exist in the same location.
6. Now you can close all the windows and launch Moho. Depending on what you installed, you should see the new tool in the Tool panel, or see the new Menu script under the scripts menu.
In your case, the tool is a Menu script called SYN: negative Keys
. Using the script is straightforward and there's more information about what it does and how it works in this thread: Find And Delete Negative Keyframes
Hope this helps.