Hi Paul,

When shelling a solid box, you need to select a face that is to be shelled.

Example:

Select shell command, then select object:-


you then select the face to shell (left click the face in any view)


Then select "done" (or right click) that will bring up the option box where you can select wall thickness and direction.


You can of course, select the face before the shell command if you prefer.


Hope that helps,

- Steve