You don't assign it directly to a texture. You assign it to the pipeline, i.e. you enable your shader and it's used on all polygons from there on until you disable it. To use a texture in a shader, it has to be assigned to a texture stage (jPCT does this for you) and you have to make your shader know it (that's your part). There are a lot of GLSL examples and tutorials that show how to do it.