Yeah, the file you downloaded will be in a .rar file..
Thats a compressed file, that contains all the needed files (Almost all addons use it, that or .zip)
www.google.com - search for Winrar
And download/install that
Then use that to extract the files..
There will be a readme.txt file, it says where everything should go
But if I remeber, the folders are something like this :
ResAddons
Addons
Missions
Copy the resaddons folder to ofp folder/res/addons
The addons ofp folder/addons
Mission to ofp folder/missions
And thats it
Winrar is easy enough to use.. Select the files you want (The 3 or 4 folders), and press Extract to... Give it a folder (Say, click the desktop, and in the file path, type Lowlands/)
And that it
Gd luck
- Ben
Oh, the files that go in the addons folders are all in .pbo files
Or islands come with a folder you also extract to the addons folder, but thats not really importarnt to you atm