first, I recommend you to use the Arc Hydro Groundwater (AHGW) from Aquaveo, which enables you to simply import the simulation results (including Draw down) into ArcGIS. If you don’t want to do that, you can follow the following tedious steps:
1- Go the project explorer- 3D grid data, and select the “top” in Global package to make it the active dataset.
2- Right click on the 3Dgird icon and then export the 3D grid as a shapefile (grid.shp).
3- In the windows of Export 3D Grid to shapefile select the “Active cells” and “Active dataset”, which in your case is the top elevation of the layer.
4- Once again, right click on the 3D grid icon and choose the commend “Convert To 2D Grid” and write 0 as the default Z value for 2D grid.
5- Now go to your MODFLOW Solution and select the DrawDown to be the active dataset.
6- Go to the Menu Bar and Select the “Grid” which is located between “Display” and “MODFLOW”.
7- Choose the commend ‘’3D data -> 2D data”.
8- A window will appear, change the option of “create dataset using”: into “value from k layer”.
9- Be sure to see DrawDown:all near the button 3D Dataset in this window, then click ok.
10- Accept the default dataset name “lay1_DrawDown”.
11- Now you have two dataset under your 2D Grid, you can delete the default, and select the lay1_DrawDown to be the active dataset.
12- Right click on the 2D grid and choose the commend “Convert to 2D Scatter Points”. You can change the name if you want.
13- Now you can choose which time step you want to export to ArcGIS.
14- Right click on the scatter points (which represent the DrawDown as points), and then choose the commend “Export..”, to be saved as type “Shapefile(.shp)”.
15- Now, you have 2 shapefiles. The first one is the Grid as polygons, and the second is the drawdown as points.
16- Import these two files into ArcMap.
17- In the table of content in ArcMap, right click on the Grid, and choose the commend “join and Relates” / join..
18- In the field of “Choose the filed in this layer that the join will be based on”, choose the field “ID”.
19- In the filed of “ choose the table to join to this layer, or load the table from disk” , choose the drawdown, which is a point-shapefile.
20- In the field of “choose the filed in the table to base the join on” choose the field “NUMBER”. Then click ok.
21- Now you can display the drawdown, by right click on the Grid, then choose the Properties.. then Symbology, then Quantities , then choose the Value to be “Lay1_DrawD”.
22- You can choose the number of classes and the color ramp you want.
23- If you have a large number of cells, you may need to click on the button Classify, then increase the value of Maximum Sample Size, through the button Sampling in the Classification window. You have to increase the value to a number that is larger than the number of the cells in your model. If you don’t increase it, you will notice that not all cells are displayed.
I hope this will help you.