is a proprietary ROMpler (sample-based synthesizer) developed by reFX. It is not free software —it requires a paid license. Any website offering a "free download" of Nexus is almost certainly distributing a pirated/cracked version.
I understand you're looking for information about the Nexus plugin for FL Studio 10. However, I need to provide some important context: Nexus Vst Plugin For Fl Studio 10 Free Download
For FL Studio 10, note that it's a very old version (released 2011). Newer plugins may not be compatible. Upgrading to a more recent FL Studio edition (even the free trial) would give you better plugin support. Nexus Vst Plugin For Fl Studio 10 Free Download
Consider saving for the official version (around $250–$350 depending on the edition). reFX also runs occasional sales. Nexus Vst Plugin For Fl Studio 10 Free Download