Never launch arma3 tools from Arma3Tools.exe launcher, always use short cut to desktop or from batch file. You need to run one of the tools to activate registry entries, for example start Addon Builder once.
To complete your installation see Setup P: Drive.
Never ever run Steam client → ArmA 3 Tools → Prepare virtual P: Drive, it will delete all your files in the P: drive if it already exists, this is very dangerous command, please be careful not to ever run it.
Any ArmA 3 steam tools updates will delete your registry and you must run the arma3 steam tools GUI menu (steam → arma3 tools → PLAY) and addon builder one time, just start the gui, click addon builder, then exit from both hideous softwares. Now your registry has been restored.