from the error, it looks like you are using DW CS6 and CSS From builder.
CSS Form Builder has not been updated for compatibly with DW CS6.
At this time, the only extensions we have that are compatible with DW CS6 are Design Extender, Data Bridge and eCart 5.
If you need to use CSS Form builder in DW CS6, I would strongly urge you to upgrade to Design Extender which includes form builder and 7 other extensions.
we will be evaluating the stand alone extensions in the coming months for compatibility but with 30 extensions, it will take some time to go through them all.