I know that this is a really old thread, but I think I can help. If I understand the problem correctly, there is an issue with setting a default coordinate system while creating a script tool. If anyone is still having this problem in 2018 here is the solution...
1. In the parameter tab of your script tool you will want to select the coordinate system parameter that you are trying to edit
2. Click the "default" text box in the editing portion of the parameter
3. Type the WKID number of the coordinate system that you want to use as the default coordinate system
You can find the WKID numbers here:
***Coordinate Systems ID's ***Projected ID's
So for example, if you wanted to use NAD 1983 you would type in the number 4269 into the box. Click "apply" and Esri will automatically populate the coordinate system in the format that it likes. Hope this helps someone out!
-Brack