You can find your Serum plugin files at the following locations:

OSX

AU: Macintosh HD/Library/Audio/Plug-Ins/Components

VST: Macintosh HD/Library/Audio/Plug-Ins/VST

AAX: Macintosh HD/Library/Application Support/Avid/Audio/Plug-Ins/

WIN

VST: (specified by user at time of installation)**

AAX: C:\Program Files\Common Files\Avid\Audio\Plug-Ins

** We recommend installing your VSTs here: C:\Program Files\Steinberg\Vstplugins\