If you click on a 2D plot, the plot toolbar should show the (cartesian) coordinates of the current position of the cursor. 

One of the new features of Maple 12 is the ability to have Maple capture those coordinates from an embedded Plot component.  That takes some work to set up.  For example:

Insert a Plot component from the Components palette. 

Right-click the Plot component and select Component Properties

In the Plot Expression field, enter some 2D plotting command, e.g.

   plot([(t+1)*cos(t),(t+1)*sin(t), t = -2*Pi .. 2*Pi]);  

Click on the check box for  "Make "execute code" the default manipulator".

Click on the Edit button for "Action When Clicked".  Before the "end use;" enter

  Do( P = [%Plot0(clickx), %Plot0(clicky)]);

(assuming the name of this Plot component, as shown in Component Properties, is Plot0).

Click OK to accept this edit, and OK again to accept all changes.

Now after clicking on a point in the plot, the variable P should contain the x and y coordinates of the point where you clicked.