The Lightroom preset folder is by default located in C:\Users\\AppData\Roaming\Adobe\Lightroom\Develop Presets.
So unless you have followed my guide on how to move your preset folder you should just download the desired preset and extract it to that location.
Start up Lightroom and go to develop module and use your desired preset. If you have moved your preset folder just copy the preset to the new location on the external harddrive.
In my case this would be F:\Lightroom_presetfolder\Develop Presets.
Now if you got a huge amount of presets you should consider sorting them into sub folders to easier locate them inside Lightroom.
Just right click your mouse and click new folder then call it what ever you desire.
There is lots of cool and free presets on the internet. I also have a few free presets here on my blog.