The middle-align tool mentioned above works to get the half-way point of the square (to draw the first horizontal line), but it won't dissect it a second time. Instead, try this:
1) Draw a second horizontal line right over the bottom side of the square
2)Draw a third horizontal line anywhere between the middle line and the bottom line.
3) Select all three horizontal lines and choose "Distribute vertically" from the Arrange tool mentioned above. When three horizontal lines are selected, this will always bring the middle one to the half-way point between the top and bottom lines.