The imperviousness is stored in another table.
I use the Time-Area(A) method and in my case I find it in the table msm_HModA and in the field ImpArea.
My suggestion is to join the catchment layer to the table msm_HModA so that you get the field ImpArea in the catchment table.
1. Switch to “Layout and Advanced GIS”
2. Add the table msm_HModA to the TOC
3. Right click on your catchment layer in the TOC and choose Join.
4. Use the catchment names to base join on, i.e. MUID in the catchment table and CatcID in the msm_HModA table.
Lable the layer as usual.