What path did you set up for the “ngram data directory” under Tools -> LanguageTool -> Options in OpenOffice? Did you restart OpenOffice after setting the path?
Are you sure you’ve set the text language (in the Character dialog) to English? Do other LT rules work? Do the ngram rules work when you use the stand-alone software (the *.zip you can also download from https://languagetool.org)?
I downloaded and unzipped the n-gram data into D:\ngrams-en-20150817. In LT standalone, I selected the ngram data directory. After I clicked ‘Open’, I got an error message (below). LT added ‘en’ to the folder name that I selected.
Mentioning this requirement of an extra en folder on the wiki would be helpful. I’m using the latest languagetool extension with LibreOffice and spent around 30 minutes trying to figure out why n-gram didn’t work before reaching this post. There’s no error message in the version 3.5 I downloaded. I’d like to contribute to the wiki but I don’t have permission.