In Poser, click on File on the Taskbar above to open the contextual menu, navigate down to Import and select Wavefront OBJ. On the Import Options window, either select one of the two radio buttons at the top for 'Of original size' or 'Of standard figure'. Without knowing how large the actual model is, you can reduce or increase the scale by entering a size in percentage. There are some other options that may or may not apply, you just need to try them. Once you have the model in the Poser scene, select the props category, create a folder in the Library and save it into your runtime. All the icon features can be found towards the bottom left of the Library panel. Be aware, according to the readme instructions, there are many other OBJ files containing other parts to the model that need to be imported also to complete it. Decals and colors need to be applied separately.