There is no fbobjclldb.py module which is mentioned in the README.md

[Liiiszt]

In Custom Command, "Chisel has a python function just for this, loadCommandsInDirectory in the fbobjclldb.py module."
Actually, the name of the module is fbchisellldb.py or fblldb.py.

[duraki]

@Liiiszt this is mistake in official documentation.

This should work:

# => make sure to import fbchisellldb.py from libexec at the very top
...
script fbchisellldb.loadCommandsInDirectory('/Users/$USER/.lldb/your_commands/src/cmds/')

Paging @kastiglione to notify about the wrongfully stated documentation.