How do you program wrobot flightclass to use a custom macro you made in-game? I can't seem to figure it out. 

+ to this question.

RunMacroText("MACRO") doesn't work at Vanilla, but on BC is ok.

need the solution

You can execute Lua directly in the fightclass (with the option Is Lua, not spell).
Whatever you use ingame, unless it's a slash command (basically anything you can put after /run or /script is possible) works with that too.

how it is possible to do?

When I try:

RunMacroText("CastPetAction(1)") or

RunMacroText("/script CastPetAction(1)")

I get this error. Option "Not spell, is lua script" is True.

Yes, because like I said, anything after /script works directly in Lua.

CastPetAction(1)

is perfectly valid Lua

I uderstand. RunMacroText() doesn't need.just plain macro text in spell tab. TY!