Thanks for the responses. Just a quick post to help someone with a similar question.

Copy & Paste the following into the command line.
filebot -script fn:configure

You will be prompted for username and then password. As Kim pointed out, these are stored locally.

You will then be able to use ...

Hello,

I have looked but didn't find the answer.

On OSX where should the config file be placed?

Is the config file just a .txt file?

Should the opensubtitles account info be something like

username
password

Any help is appreciated.