Make sure you have the latest version of Model Builder: Version 1.6.11 works with the browsers better than earlier versions.
If you do not have the latest version or you are not sure your vesion number, check here: "http://www.modeltrainsoftware.com/modelbuilder.html"
Close Model Builder before you open any file.
Opening files varies depending on your browser.
Click the file name. You will see a window pop up that asks "do you want to open or save this file"
Choose "Open" the file will open in your copy of Model Builder automatically.
Click the file name you want to download. You will see a box that asks "what should Firefox do with this file"
Choose "Save File"
Firefox will give you the choice where to save the file. When the download completes, double click the file to open it with Model Builder
click the file name you want to download. You will briefly see a blue and white arrow on the left corner pointing downward. Then the file name
will appear in your file toolbar. Click once on the filename to open the file in Model Builder.
You can open any file found on these pages in your scale. Download the file and save it to your computer. Open Model Builder in your scale. Then
select "File" > "Open" and browse to the file you saved. Click "Open" and Model Builder will ask if you want to open in the new selected scale.
There is more detailed description of this under the "Tips" tab.