Midpoint

1. Start with two points, $P_1$ and $P_2$.

2. Draw a line segment $\overline{P_1 P_2}$

3. Draw circles $\overset{\frown}{P_1 P_2}$ and $\overset{\frown}{P_2 P_1}$. Note the intersections of the circles.

4. Draw a line segment connecting the intersections of the circles. Where it intersects line segment $\overline{P_1 P_2}$ is the midpoint.

AlDraw also has a shortcut for finding midpoints. It is under the "Shortcuts" menu, and its hotkey is alt-1.