The downloaded file is a code-signed XLAM addin file.
Copy the XLAM file from your Downloads folder to a folder of your choice.
Unblock the XLAM file (right-click the file, choose the Properties Tab and Unblock).
Use the Excel Addin Manager to install or uninstall Lambda Explorer.
Or for single-session use just use Excel to open the XLAM file.
When successfully installed in Excel you should see this Lambda Explorer Tab
If the Lambda Explorer Tab is not visible make the Lambda Explorer Folder a Trusted Location
Use the FastExcel V4 Google Group to report problems and request improvements