A couple of navigation questions

1. how do I send a landmark to a friend?
2. how do I visit a location by typing in it's map coordinates?

1. Open friend's profile and drag the lm from your inventory onto the profile.
2. Open the map. The sim name is put into the box to the left of the search button. The coordinates are entered in the 3 boxes below the search results box

An landmark is just like another inventory item. Just drag it from your inventory either onto their profile or the avatar itself.

The map coordinates can either be used it the map or by typing http://surl.com/secondlife/<sim name>/<x>/<y>/<z> in open chat and clicking the link.
There are couple of other ways, but that's the simple one.