I recently bought the De-Esser for our SQ5. Please excuse a dumb question but what do the red meter lights actually show?
These meters show the reduction of the occurring S-sounds depending on the set parameters.
But because you ask this, your De-Esser may not yet work as expected.
Remember: It is typically used as an insert effect (e.g. in a vocal channel or group) and not as an FX send and return.