There are several Lightroom namespaces that are not importable, LrFolder is one of them.
You just obtain lr-folder objects from other objects, then call methods on the lr-folder objects without importing the namespace.
There is a complete list of non-importable namespaces at https://www.assembla.com/spaces/lrdevplugin/wiki/Lr_-_do-not-import
Thanks for help Rob. I'll take a research about that.