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.