Davey,
Make sure the current renderer is set to 3D Raytrace.
Steps from the helpfile:
1.Create a new composition.
2.Open the Composition Settings dialog box for the composition.
3.Click the Advanced tab, then set the Rendering Plug-in (now called Renderer) to Ray-traced 3D.
To turn an existing composition into a Ray-traced 3D composition, omit the first step.
Your composition is now a Ray-traced 3D composition that allows for extruded text and shapes.
Hope this solves your issue.
kind regards,
Maarten