Open the Grid Manager and enter the following line in the Login URI:
http://login.ngrid.org:8002
Now you can enter the Username and Password (like in this example).
If you do Hypergrid jumping (from one virtual world to another):
hypergrid address: hg.ngrid.org:8002
(Old version) This PDF document shows how to configure the Grid Manger in your OpenSim Viewer: http://ngrid.org/files/login-manual2.pdf