A commend mistake is looking for the plug-in in the mix section, make sure you add the plug-in in the right way.
FL Studio uses two different types of plug-ins. Effects such as reverb or compression, and virtual instruments like drums or synths. If you try and add it to the incorrect spot, the plug-in will not show up. Follow these steps to load it to the project correctly.
Click the ‘+’ button on the bottom of the channel rack, then select the virtual instrument you’d like to load.
Navigate to the Mixer window and select the track you want to add the effect on. Click on an empty slot, then select the plug-in. You can't load virtual instruments here.