Child process exited with status 254 error when extracting a tar.xz file
Same error here (tried GNU tar and the one included in OS X). 

You are missing the xz compression program which tar tries to use.

Either install a precompiled xz from Sourceforge or, if you are like me, download the source from http://tukaani.org/xz/ and compile it yourself:

$ ./configure
$ make
$ sudo make install


(Unless told otherwise, ./configure installs xz in /usr/local/bin. You possibly want to add it to $PATH.)

Then:

$ xz --decompress cclive-0.7.11.tar.xz
$ l cclive-0.7.11.tar 
-rw-r--r-- 1 jaume 1832960 2012-12-12 15:18 cclive-0.7.11.tar


Now untar as usual:

$ tar xf cclive-0.7.11.tar