Using OS 10.9.1. Using LibreOffice 4.1.4.2. I have been using LibreOffice for about 4 years now and I’ve never discovered the secret of installing a Thesaurus (English). I’ve looked online and searched but nothing has succeeded. I’m quite prepared to accept that I’m a dunce but if it is possible I would be grateful for some assistance. Many thanks.
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] Under MacOS the leading /opt/libreoffice4.1/ is /Applications/LibreOffice.app/Contents/.
Thanks for the help but that’s way beyond my comprehension. I double click icons!! 
Which thesaurus are you trying to install? The en-US one is included in the default install as far as I know. There are very few thesauri available.
There is a list of files in the Extensions Manager window. One says, "English spelling dictionaries, hyphenation rules, thesaurus, and grammar checker. 2011.12.05.1
Another is, "British English Spelling, hyphen, thesaurus 683k word list. 2011.11.09
There is also a French and a Spanish file and a couple of others relevant to other parts of Libre Office.
Thanks for your help.
The en_US, es_ES, and fr_FR thesauri are installed by default. The en_GB one is a separate download. In what way are they not working? Do you not see the Synonyms entry in the context menu?
Please disable (or remove) the “British English Spelling, hyphen, thesaurus 683k word list. 2011.11.09” in the Extension Manager. Then restart LibreOffice.
In LibreOffice → Preferences → Language Settings → Writing Aids → Available language modules:
(1) 
OpenOffice.org New Thesaurus (enabled)
(2) Click the Edit… button, select the required languages, and enable
OpenOffice.org New Thesaurus
for the required languages.
Restart LibreOffice.