I am working for a client and the platform we are using is vBulletin. I'm so frustrated with its almost zero documentation and non-existence in Google. We don't have resources and all we have is the source. I almost want to give up now and get a job which I am familiar and uses tools that are open source.
My first thought would be to look at the online support options:
Your client (if they are a paying customer) should give you access to their support contract/account so you can ask questions and be more productive for the client.