To add a fuel record, check a vehicle is selected in the filter at the top. The New button won't be active until an individual vehicle is selected. I'll add text beside the buttons to explain this. All of the panes will eventually work like this where relevant. For example, in the future, the New button on Tires won't be active if "All" is selected, as that makes little sense.
I'll make this one as "not a bug" but if I've got it wrong please let me know.