You should just need to add a new layer, which covers all the scenes in the time line, and put some code in it to check if the
my_key = Key.getCode();
if (my_key = <code for back key>) { prevFrame(); }
if (my_key = <code for fwd key>) { nextFrame(); }
The code for space bar is 32 but I'm darned if I can get a key code for backspace (just put a trace(my_key); into the code above, run the movie and then hit keys to find the codes!).
Alternatively, of course, you could use the standard Flash keys which are comma (,) for back a frame and full-stop (.) for forward a frame!
=)
PetitPal.