Any code you wish to use should be saved as a .vbs file. Putting it in the same directory as your .gdp file is a good idea. Once you've created the script, connect it to the game from the settings dialog accessed by pressing the "Key" button in the toolbar at the top and browsing for your script in the Script box.
You will, of course, have to customize almost any script to point to the objects (maps, sprites, etc.) that you are using. Here's a script to get you started (found elsewhere on the forum). This will take the first sprite defined on the first layer of the current map and move it with the mouse. Change MapLayer(0) to MapLayer(X) where X is the numerical index of the layer your targeting reticle is on counting from 0, from the top down, and change Sprite(0) to Sprite(X) where X is the numerical index of the targeting reticle sprite on that layer. If it's the first or only one, leave it at 0.
Sub Display_MouseMove(Button, Shift, X, Y)
.rMap.MapLayer(0).Sprite(0).X = X + .MapScrollX - .rMap.ViewLeft
.rMap.MapLayer(0).Sprite(0).Y = Y + .MapScrollY - .rMap.ViewTop
HostObj.SinkObjectEvents CurrentDisplay, "Display"
HostObj.SinkObjectEvents ProjectObj.GamePlayer, "Player"