You need to select your iPod in iTunes. The command (from the menu bar?) is disabled because you have not selected a device for which the command is applicable.
The latest iTunes may hide the sidebar. To show the sidebar, from the menu bar, under View, select Show Sidebar. In the sidebar, find and select the iPod under DEVICES.
Once the iPod is selected, you can select the Sync command under File -> Devices in the menu bar. You can also just click the Sync button at the bottom left corner of the iTunes window (when the iPod is selected).