External paths are listed in the 3ds Max configuration Customize - Configure User Paths - External files stored in 3dsmax.ini, by default located at:
C:\Documents and Settings\<username>\Local Settings\Application Data\Autodesk\3dsmax\<max version>\enu\3dsmax.ini
Generally, we recommend to resolve all references used by Forest Pack / RailClone with the Asset Tracker to absolute UNC paths. It's not necessary, but usually this way you avoid any texture path issues while using distributed rendering.
Anyway, might be, that's not a paths related issue and thanks in advance for any further comments on this issue you can share about it with us / our user community (in case some user(s) will run into similar troubles).