(Using Windows 7 Professional - 64 Bit; Civil 3D 2013)
No changes have been made to the pressure network catalog (default, as installed and located in the default location on each machine).
So, I begin by setting the pressure network catalog and choosing any of the sqlite database files in a new blank drawing. In the "Create Pressure Pipe Network" dialog, I try to edit the parts list, but an error shows up and says "This Pressure Parts List references a catalog that cannot be found. As a result, you may be unable to add parts to the network.". Everytime I set the catalog, Civil 3D seems to immediately forget which sqlite file was selected. This problem has been encountered on multiple workstations in our office. Only 2 workstations retain the catalog selection and will allow a pressure network to be created with no issue concerning the selected catalog.
Ok I just had this happen and I am trying to figure it out.
In my case our IT guys cleaned up our server. We had to places where we stored all of our shared setup stuff for Civil 3D, Pipe networks and Pressure networks were in those places. So my IT guys moved everything to one spot. They told us before they were doing it and just after they did it. So I remapped my Civil 3D and now I can not access the Pressure network catalog. I have remapped it in both the Options and in the location under the Home tab where I found it. I can find the location and select Mechanical or Pushon or Flanged and say ok. But after that when I go into a drawing that has an existing pressure network and try to modify or change that network, I get a "can't find the pressure network catalog" and the rest that you mentioned. It's like after you copy it to a new location and remap it just can not relocate it even though you pointed it there.
If I figure it out I'll let you know how I fixed it.