You'll need WinZip or WinRAR to extract the files from Tactics.zip. You can get Winzip from
http://www.winzip.com, not sure where you can get WinRAR. After downloading and installing WinZip/WinRAR, right-click on Tactics.zip and unzip that folder to your BG2 directory. (Most likely C://Program Files/Black Isle/Baldurs Gate II is the path.)
Once Tactics.zip unzips, find a file in your BG2 directory called setup-tactics.exe and run it. You'll be brought to the WeiDU install screen. Follow the directions--it's simple enough.
Then load up BG2 and see if it works. [img]smile.gif[/img]