Okay, if you're still having issues exporting, why not start with something simple. Follow along and we'll see what happens.
Let's start a new scene and create a flat plane.
Click on the Create tab, make sure the Geometry button is selected. Select the Plane button under the Object Type rollout. Expand the Keyboard Entry rollout. Assuming your Units are setup properly set your Length and Width to 1000 and press the Create button.
Press the Zoom All Extents button to get a good view of your mesh.
Right-click on your mesh and move the mouse over Convert To: and then select Convert to Editable Mesh. Now we'll throw a texture on the mesh.
Click on the Material Editor button to open the Material Editor dialog.
Click on the empty box next to the Diffuse color box this will open the Material.Map Browser.
Double click on Bitmap and browse to the texture of your choice. After selecting your texture you'll see one of the Material Slots filled in with your texture.
Make sure your mesh is selected. Click on the Assign Material button then the Show in Viewport button. You should now see your texture being displayed in the Perspective viewport.
Now back to the Create tab, select the Helpers button. Click on the AuroraBase button to open the Model Base Wizard dialog
Select tile from the Model Type dropdown, check Add standard helpers and Center to [0, 0, 0], name it anything you want and press OK. Select your mesh and link it to the model base you just created.
With your mesh selected, press the Select and Link button. Press "H" on your keyboard to open the Select Parent dialog
You can double-click on the model base or select the model base and press the Link button. Once again select your mesh.
Click on the Modify tab. Open the Modifier List dropdown and select Aurora Trimesh. You have now added the Aurora Trimesh modifier to your mesh. From the MDL Trimesh Parameters make sure Render is checked. Select your model base.
From the MDL Base Parameters rollout, Browse to select your Export Directory. Press either Export Model + Anim or Export Geom Only.
In your chosen export directory you should find two files, one with a .mdl extension and the other with a .wok. MDL is your model file, WOK is the walkmesh file for the model.
Make sure your texture is in the same directory as your model files. Open NWN Explorer and press the open button, browse to where your exported model and texture is. Select the MDL file.
Now because my tile is a flat plane I had to adjust the camera angle in order to see it.
That is the basics of creating a tile for NWN. There are quite a few things I did not cover like UVMapping, Walkmeshes, animations and getting it into the game itself.
First I want to make sure that the version of Max your are using with NWMax Plus is working properly to export your models, not to mention just making sure you are doing the basics properly.
HTH
MDA