The adding of effects in your effect chain takes place in the FL Studio Mixer. The purpose of an effect is to monitor or tweak your audio signal. When you engage each Insert slot in the mixer, you have the option to add up to eight effects in your chain. You can always add more effects by creating submixes to further mixer channels.
In order to start using effects on your mixer tracks, you have to have your audio signal sent to actual mixer inserts. This is reviewed in the Sending a channel to a mixer slot recipe in Chapter 3, Working with Step Sequencer and Channels.