Models are made and edited by special 3D modelers programs (e.g. 3DSMax). You can add them to a map using the entity "misc_model" in GtkRadiant; when you place such entity it will ask a path to the model (.md3 or .ase files); select the model file in your HD. Of course you need to find the file by itself in the disk, i.e., it can't be inside some .pk3 or .zip.