Hmm, some areas are still not documented, I know, sorry
Instead of going through sources I recommend to check my test script. AFAIR everything which is implemented is covered in this script, because I use it for my own testing.
Switching between Predefined and Scripted tab is not only GUI thing but valid option how macro behaves when triggered. If you choose Predefined and pick some option (like Run application) then you define what macro will do. If you switch to Scripted tab then macro will run a script when triggered. Y...
In HidMacros download there's help rtf file describing syntax of sendkey command. LuaMacros use the same code. I should make it online, I know :-/.
Anyway it seems you have answered yourself or this syntax doesn't work in LuaMacros?
Installation LuaMacros can control Xplane as my main motivation is flight simulation. You need to install Xplane plugin to get interface to work. Xplane plugin is currently provided only for Windows platform 64bit version. LuaMacros is windows specific program so there's no sense to provide plugin ...