Hi,
You might have previously marked the 'Do not show this message again' checkbox to disable the pop-up window. You can revert it to its default setting (to show the pop-up) by setting CheckTexturePath = 1 in the following system registry key:
[HKEY_CURRENT_USER\Software\Itoo Software\RailClone Pro]
Optionally, you can use the attached file (RC_CheckPaths.reg) to restore it.
It's checking folders specified in the following registry key;
[HKEY_CURRENT_USER\Software\Itoo Software\RailClone Pro\TexturePaths]
These paths should appear in the 3ds Max settings in the 3dsmax.ini file. By default it is in:
C:\Users\<yourUsername>\AppData\Local\Autodesk\3dsMax\<max version>\enu\3dsmax.ini
I hope that helps.
Best regards,