Re: NIssan NS25C3 Shifting into gear
I would start with a Parts Catalog, available from
http://www.internetoutboards.com/PartsCatalogMenu.htm. That will show the geometry of the shift linkage, which will tell you a lot. Also, as TG mentioned, the engine should be running, or at least cycled by hand to allow the clutch dog to shift into gear.
I have had 2 different "it won't shift into gear" motors come in recently. Neither of them had any gear issues. One had overheated so badly that the plastic shift handle had melted around the shift detent. The other had a broken lower motor mount, which allowed the motor to come forward against the transom, jamming the shift linkage.
Right now you have a symptom, but need more information to diagnose and repair the issue. So get the parts book and review how it is all supposed to work. Then we can help you diagnose the problem.