Here are insructions with pics to help you out.
1.
Make sure the ZTD file is in the dlupdates folder before you begin.2.Right click on the ZTD file you want ZT to open you should see a menu like this
3.Click on Open With and you should get this window
4.make sure you check the box I have circled in the pic and click on the Browse button - You should now have this window
5.Navigate to the Microsoft Games folder and click on Microsoft Games folder to highlight it - click Open and you should now have this pic
6.Click on the Zoo Tycoon folder and click Open - you should now have this pic
7.Now locate the Zoo.exe file and click on it ti highlight it and click open. Your ztgame should start the file should be visable in the proper ZT menu for objects, animals, etc.
if you just get this screenScroll down and click on Properties
This window will open
Click on Change - you now have this window again
just follow from step 5 above to change the program
I hope this tutorial has helped