Hello,
-Make sure Mail > Preferences... > Junk Mail > Enable junk mail filtering is enabled - this will also add the junk mail button to the toolbar.
-If you don;t see the junk mail button: View > Customize Toolbar in the menu bar and drag the junk mail button or whole default toolbar to the toolbar area of the window and click Done.
-You can also Control-click/right-click on a message and select Mark > As Junk Mail from the contextual menu that appears.
Let us know if that doesn't work,
C