Yes, the SDLPROJ file can be double-clicked to open a project in SDL Trados.
External CAT tools (not just CafeTran) can only handle the SDLXLIFF files or packages with SDLXLIFF files that contain the actual translation segments.
Please ask/use SDLXLIFF files or SDLPPX packages instead.
Status "Not translated":
- Since you have access to SDL Trados, you can filter these segments and remove the target text, before opening the files in CafeTran.
- Alternatively, in CafeTran, you can use Task > Remove target segments same as source to remove the content of target segments that just feature the source text. Then proceed with the translation of empty segments (by filtering them or by using the shortcut for Action > Add segment to memory and go to next untranslated segment). I also suggest you back up your files before doing so.