Hey Tom,
I had some issues installing the tools from steam. Found the following causes:
1) In HKCU/Software/Bohemia Interactive/arma 3 tools/Path there was a space and a parenthesis at the end of the path causing the creation of the p-drive to fail
2) DevP is not checking the steam registry keys for the Arma 3 installation path (as in line 114 and 116 of the mapfolders.bat), thus %ArmAPath% is incorrect
3) When Arma has not been installed in the default path, even adding the check to DevP still would not help
After fixing the registry key entry and hardcoding %ArmAPath% inside both batch files (DevP and mapfolders), the installation worked for me... the scripts would probably need a re-write, since there seem to be quite a bit "backward compatible" loops inside, which are not working / not valid for the steam edition.
Cheers,
Michael.