I'm assuming you are referring to the Open Journal program, so I'll go on that basis. If you're referring to POD, it is much the same thing.

In what sense do you mean change your username/password? Do you mean change from one to another, or change an existing one to a new one?

When configuring OpenJournal I spent hours trying to figure out why I was getting a 500 error. I had set up everything exactly as John instructed but nothing was going right. In the end I had to ignore John's instruction about setting certain directories/files to 777 and set them all to 755. Everything worked fine from then on.

To add a new entry, go to http://www.yourserver.com/cgi-bin/oj.cgi (or equivalent). Log in and do your thing.

To view your open journal, go to http://www.yourserver.com/oj/ (or equivalent)

Hope this has helped.