I am trying to commit some changes back to an svn+https server. It keeps failing. I know it needs a particular username and password, but I can't figure out how to make dolphin use it. Does anyone have any ideas?

The username is usually encoded into the checkout itself, causing the password to be prompted for. Try checking out using a url as follows: https://username@server/path/to/repo/

This should ensure your username isn't forgotten, and should allow the plugin to prompt for the https password when needed.