If the effect is Chorus Effect then please see Missing features - Audacity Support. Note that the author does not consider this plugin ready for release. Other Chorus plugins are available online as VST effects. You would be able to preview a VST effect in real time while adjusting the controls, but not so with the Nyquist Chorus effect.
If you need more help, please remind us what chorus effect you mean and tell us what version of Audacity you have (all three numbers, see the pink panel at the top of the page).
Gale