Manually adding the folder should work so what is the specific issue you are having with it? Is 'Enable Hand History' selected in the Merge client under 'Settings-> Hand History Settings'? If it's enabled does the hand history folder exist on your computer to add? The default location is:
C:\Users\[Your Windows Username]\AppData\Local\[Your Site Name]\history
If you are not seeing the AppData folder when searching for it then Windows will be hiding that directory so to make it visible in Windows File Explorer please refer to
this guide.