What is your OS and DW version?
When you start the Checkout wizard, what access point are you using? WebAssist -> eCart -> Checkout wizard?
or are you using the insert panel? try using the top level WebAssist menu.
also make sure you have the latest eCart version installed. 6.0.6, if you have a previous version, make sure to uninstall it from the extension manager before installing the newer one.