I was right, what you want is easy.

The triangular "Select" functions might be unfamiliar. The help describes them this way:
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Returns the value wired to the t input or f input, depending on the value of s. If s is TRUE, this function returns the value wired to t. If s is FALSE, this function returns the value wired to f.
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So when neither button is pressed, the motor is set to 0. When the forward button is pressed, the motor is set to 1 (full speed forward -- change the constant if you want some other speed). When only the reverse button is pressed, the motor is set to -1 (full speed reverse -- change it if you want).