I think I started a thread on this subject on the old forum, but here goes!
If it's anything like the N16, you have to cut your old knob off (oo-er missus!!!) and then use a spanner, or socket (19mm I seem to remember, but could be wrong??) and remove a very long sleeve nut from the gear lever. You'll probably have to remove some old glue from the shaft, as it does seem to run down the shaft (Nissan are obviously a bit over-generous in this department) and possibly rub your shaft down with a bit of wet and dry.
(Hopefully, the moderators won't censor any of this!!)