Hm... This is for a PC (Windows) and maybe my translation for the terms are not the perfect. Anyway, a setup should attribute skp (and skb) files to SU by default. So right click on and skp file icon and choose "attributes" (maybe this is different - but somewhere bottom-most of the context menu) > choose the settzing that is shown in the attached image with the cursor pointing at (I have no idea what the English system would translate that) and change the attribution to SU (you can navigate to the application (C > Program Files > Google > Google SU6 > SketchUp.exe) > select and hit OK (or what). Now it should open any skp files with SU. If you are a Mac user, please say so.