There are two ways to install the ViXL-3D add-in into Microsoft Excel. The easiest is to simply double-click the file ViXL3D.xll located in the ViXL3D folder shortcut that should have been installed on your desktop.
You should also be able to find the shortcut in the Windows Start Menu (or Start Screen on Windows 8.x). Inside this folder you should find the file ViXL3D.xll (along with several other documents and sample spreadsheets).
When you double-click this file, Excel will launch automatically and install the add-in.