The encrypt system partition or entire system drive feature is not available in most modern operating systems due to newer boot and partitioning structures. Specifically, I don't believe UEFI or GPT are supported. It is also possible that the feature was never supported on OSX regardless, but I simply don't know about that. Regardless, to answer your second question, outside of the system partition/drive feature that requires a decryption password to boot, you don't encrypt files in place with TrueCrypt, you place files on a volume that was created with encryption already in place. You can create a volume on an empty disk (not recommended because there several things can go wrong when the disk appears to be empty), empty partition, or as a file container. After it is created, you mount it and transfer files to it.