The DAT and IDX files are placed in the share/extensions/dict-xx directory. There is a reference to the DAT file in[1]:
$grep -re th_en_US /opt/libreoffice4.1/ /opt/libreoffice4.1/share/extensions/dict-en/dictionaries.xcu: <value>%origin%/th_en_US_v2.dat</value>  Have you tried that? [1] Apart from the leading /opt/libreoffice4.1/ the directory structure under MacOS is the same as that for GNU/Linux. The DAT and IDX files are placed in the share/extensions/dict-xx directory. There is a reference to the DAT file in[1]: $ grep -re th_en_US /opt/libreoffice4.1/

[1] Apart from Under MacOS the leading /opt/libreoffice4.1/ the directory structure under MacOS is the same as that for GNU/Linux./Applications/LibreOffice.app/Contents/.