Hi Bruce,
Customization of toolbars can only be performed in ArcPad for Windows and not ArcPad for Windows Mobile therefore Design Mode is not available on mobile device. I will work with the ArcPad team to add this information do the Help documentation in 'Working in Design Mode' topic. Thank you for bringing this to our attention.
Hopefully you have discovered that making ArcPad Apps (with custom toolbars) is available in both ArcPad for Windows and ArcPad for Windows Mobile environments. If you wish to use custom toolbars in your Windows Mobile Apps, you must first customize the toolbars on Windows, then copy them to your Windows Mobile device.
Cheers,
Denise