This depends on where you downloaded the mod from.
If downloaded from Steam Workshop just find the Emitter 3Ditor in the game’s launcher mod list and load it.
If downloaded ye olde-skool package unpack the package and copy @HNEG_emed folder into your Arma3 folder for example, then in the game launcher “add local mod”, find and select the @HNEG_emed folder. Make sure to load the mod.