OK, I figured it out. I modified it with a partial script someone else wrote at one point...
/run local MB=UnitMana("player");MT=true;if MB>=80 then MT=false;for i=1,40 do if UnitBuff("player", i)=="Disassembly Mode"then MT=false; if not(MT)then if GetActionUsable(5) then UseAction(5)end end end end end
and it works. Turns out that when I was adding it originally, I forgot to add the extra "end" at the end. /facepalm