The catchments come in only when you have referenced them to a stormwater structure that can receive runoff- ie inlets, endwalls,etc. (of course the structure should be in the catchment). Open your catchment properties. The fourth line under Catchments is looking for a network structure.
The other thing that I edited was the pipe network settings for part matching. You go to settings on your toolspace, right click pipe networks and click to open Storm Sewers Migration and then click to the right of standard next to next to part matching to open the menu where you can tell C3D how to export and import parts between it and the storm water programs. The help file in autocad has a good list of what the parts should be changed to so that storm sewers and SSA can interpret the info correctly. If for instance all of your part matching says that your inlet and headwalls are matched to a form of structure that can't receive water...manholes...then SSA sees that the catchment can't catch water because it can't get in the system! Check this out and reset the defaults to a logical structure.
Good luck,
Carrol
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