NDI 4.5.1 uses classic mDNS discovery. Your firewall may be blocking port 5353 (mDNS) or TCP ports 5960-5980. Solution:
Once you have safely downloaded the file, follow these instructions for a clean installation. ndi 4.5.1 runtime download
If you are reading this because you think you need 4.5.1, but you are actually setting up a new system, consider whether you truly require this legacy version. NDI 5.5 and 6.0 offer: ndi 4.5.1 runtime download