You have to use eval. Here's how things work: hey is a movieclip on the stage, ho is a movieclip in hey.
PHP Code:
myMc = "hey.ho";
my = "hey";
Mc = "ho";
trace(hey.ho);
trace(eval(myMc));
trace(this[my][Mc]); 
Traces:
PHP Code:
_level0.hey.ho
_level0.hey.ho
_level0.hey.ho