Who , according to what romeo tells juliet , gave him directions to Juliet's balcony ?

Cupid

A more exact answer would be love, this quote from a bit later in the scene.

"By love, that first did prompt me to inquire.

He lent me counsel and I lent him eyes.

I am no pilot. Yet, wert thou as far

As that vast shore washed with the farthest sea,

I would adventure for such merchandise."