If your ladders are not working in your custom or original BIS models when you make new WRP terrain, then it means that your build environment is not setup correctly or there is problem with your custom object configs. You cannot have any “house, config class missing” errors in RPT because that is sure sign of not having properly configured objects and binarized WRP.

When making custom object configs, you must add the Land_ prefix to the class name and of course have the proper ladder parts as well, otherwise it just doesn't work.

Reference: Build environment.