Hi Rob,
on the left side you have to select the equipment model. If it is emtpy in your case it probably means that you have no equipment model in the global project.
This is the extract from the help:
Waterfall chart filter
You define the machine-based waterfall diagram in this tab. To do this, all variables must be assigned to the same equipment group. If variables from an archive are used, the archive and the variables must be assigned to the same equipment group.
Attention: Only equipment models from the global project are available.