Click the 
Cab Builder icon > select a furniture catalogue from the drop down list on the left > click the Modifiers button underneath
This is where you can set your modifiers specifically for this catalogue. Your modifer will need to have bene added into your Global Modifiers in the first place.
The advantage of setting a modifier against a catalogue is that you may have used a modifier formula to create a panel, but the value of that formula may differ dependent on the catalogue. A good example of this is Plinth Height. For most Kitchen furniture catalogues, the Plinth height is standard 150mm. However, for some catalogues the plinth may be 100mm high– Bedrooms are a good example of this. Bathrooms quite often are 200mm. This can be set here, so without the need to duplicate panels to take account of this, we can set the modifier to be specific to the catalogue. Voids are another good example of this.
If you only have 1 catalogue that’s being configured in Cab Builder Pro you won’t need to set any catalogue specific Modifiers