The trick to installing NITDroid is to make use of the "auto-complete" feature (by pressing the "Tab" that you see on the screen). This helps reduce human errors brought about by typing whole commands. I would recommend this approach even to pros/expert users as it's always possible to commit an error when typing.

For CSSU, try to refresh all your repositories/catalogs and check that you have enabled all of them. I haven't encountered any problems so far with CSSU updates. Or yes, you can uninstall that for the meantime if you want.