The servos have three connections, two for power and one signal/control line. Simply connect the power to + and – on the Crumble, and connect the signal to A, B, C or D (you can connect up to 4 servos).
The wire colours vary, but I believe the SG90 is:
I’ve attached a simple program that should test the servo (on A). One thing to be aware of is that when a program finishes, the Crumble stops outputting the signal to the servo. The attached program is looped, so never ends.
I hope this helps.