Save the file first, instead of 'opening' it. Save it somewhere on your computer like the "Downloads" folder or your desktop. At this point, it doesn't matter where this file goes.
After saving the file, go to the folder where it was saved. You should see a file called "Ext_growth.zip". Double-click on this file. Depending on how your computer is set up, this will either open an archiving program or simply open another window. ( If an archiving program opens, use it to 'extract' o 'unzip' the contents of the file, then go into the extracted folder. ) You should see a folder called "WINDOWS" go into this folder and the desktop folder underneath it, and you will see those three files.
Find the place where you installed Black and White and go to its folder. Usually, this is something like "C:\Program Files\Lionhead Studios\Black and White". Place the "ExtremeGrowth.txt" file into the "\scripts\playground" folder inside of the the Black and White folder. Place the "dance.lnd" file into the "\Data\landscape" folder.
To use this, it looks like you will have to meet come requirements for your creature first, like teaching fishing and eating grain. Read the readme file to make sure you do all of these necessary things.
-ego533