Each Basecamp "organization" or "company" that you're a part of as a unique ID. You can find this number just to the right of the URL in your browser: https://basecamp.com/xxxxxxx. The x's represent the number you want. If you are on a project page, the URL will be https://basecamp.com/xxxxxxx/projects/yyyyyyy. The x's are still the number you want.
If you are a part of several organizations, then you will have to find each organization's ID and assign them to "basecamp_company_ids" as comma separated: basecamp_company_ids=xxxxxxx,xxxxxxx,xxxxxxx.
Does that help? I'd be happy to help if you have more questions. I will also add this tomorrow to the instructions to make it a little more clear. Thanks!