Go to "Decipher an Equation" (to the left) and type in your equation*--try "f = m a". Then click "Check the Equation..." for the variable recognition step.
First, the tool tries to recognize the "textbook" form of the variables, and "f = m a" is easy; "f" is recognized as Force, "m" as Mass, and "a" as Acceleration.
Now click "Play it, SAM" for the equation recognition and analysis step.
Try the following:
You can change the question in the recognition stage by typing in different units or scalar/vector properties. To bring up scalar/vector properties, check "Scalar/Vector" in the "Show Results For:" section and click "Redisplay". If you pick "X for variable "a" (Acceleration), and click "Play ..." the vector equation is shown. Similarly for units or value.
If you check "Dimensions" and click "Redisplay", then dimensions information will appear.
* You can set the Unit and Coordinate systems, too.