First you will need the MBD module. There is an additional cost for this. If you already have it, you turn it on in the addins. Another place to look, is to right mouse button on the command menu and look for it in the list and place a check next to it.
You may need to install MBD first, to do this go into your 'Programs and features' from your start menu, find your SW installation and left click once and select change, select 'Mody' and click next, you should get to the page below
ensure SW MBD is checked and input the serial number then click next through the installation manager and install it, then right click on the command manager tabs and you should see the option to show the 'SOLIDWORKS MBD' tab