I also needed a Custom Tab in the Ribbons Tab along with Home, Insert, Layout, etc. What i did was use this syntax:

<ExtensionPoint xsi:type="PrimaryCommandSurface">
            <CustomTab id="CustomTab">
              <Group id="CommandsGroup">
                // Code 
              </Group>
              <Label resid="CustomTab.Label" />
            </CustomTab>
</ExtensionPoint>

If i put the CustomTab Label Tag above Group Tag it gives me a syntax error , but if i put it below it gives me a valid manifest file and its showing the tab.