.FXB refers to the Bank file (where all the synthesizer presets are loaded into for quick use). To load a .FXB first, load the synthesizer (Sylenth1) into your DAW. Once Sylenth1 is loaded click on the Menu button and then click Load Bank. After of which, locate the “.FXB” in your downloaded folder or wherever you have it downloaded, and it will load up into Sylenth1.