Arma Reforger:Resource Manager: Getting Started Tutorial
The Resource Manager requires game data and may not autodetect it. Click the Add Existing button in the Projects popup to target ArmaReforger.gproj, located in <Arma Reforger installation dir>\addons\data. Once this is done, Arma Reforger appears in the projects list.
See Mod Project Setup.
One of the first things to do is to register the enfusion:// protocol by using Workbench > Options > Workbench tab > Register enfusion:// protocol). This allows to click enfusion:// links such as this one: GenericProfileIcon_UI.edds - such links may be encountered on internet, and especially on this wiki.