The PAK File Tab
The drop down list on top allows you select from any Pak file that you have loaded. Note that if you have VWep support, the pak files that you set up in the VWep Option dialog will be available through this list.
On the left is a tree control (list) that starts with env, models and textures.
On the right are 3 buttons. The buttons will be enabled when you select an item in the list that can be added to your scene.
If you click on the name of a picture or texture, a thumbnail view of the will appear. This also enables the Background and Texture buttons. The effect of the Background button is immediate. The texture button adds the texture to the Scene's texture list. You will have to assign the texture to an object through the object's properties.
If you click on a model, the View Model button is enabled. Clicking it will load the model into the center of the scene.
see also Opening a Pak File