Trimble hosts the base station list here ftp://ftp.trimble.com/pub/mech/pfo/cbs_list.csv and that is where PFO and Positions are looking to download the file. The cbs_list.csv is stored on the local computer under C:\ProgramData\Common ...
The attached support note demonstrates the workflow for using Pathfinder Office to post process Positions and GPScorrect ssf files and apply the corrections to the axf file.