1) To install the "Multisites Menu Item" into the slave site, you can use the JMS Tool menu and just select the "install" action in the slave site and click on the "execute" button.
If you want to replace existing menu item with the "multisites menu item", you have to rebuild them.
2) Concerning the sharing the "/templates" directory, check the "JMS Template rule" that you created to build the slave site and verify that you have select the "special copy" action for the "/templates" directory.
See also the tutorial video 0.b slide 34 that show the different possibilities concerning the setup of the "/templates" directory.
www.jms2win.com/en/tutorial#jms12x-0b
If you want to have a specific "/templates" directory in a slave site, get inspired of the tutorial video CP-05
www.jms2win.com/en/tutorial#jms12x-CP-05