Remember that MatrixForm is a wrapper function that affects how lists display in the notebook. Once you wrap your data in MatrixForm (or TableForm), you can't work with it easily. If you did want to enter some data and display it as a matrix, but still be able to work with it, you can do this:
MatrixForm[data={{1,20},{10,20},{30,19},{40,17},{50,15},{60,15}}]
This defines the data first, and then displays it as a matrix. (I.e., it is equivalent to)
data={{1,20},{10,20},{30,19},{40,17},{50,15},{60,15}};
MatrixForm[data]
Then you can still do data[[All,2]], etc.
