How do I open Dolphin in a certain folder from the terminal?

Try this in the terminal: dolphin /usr/home/ (choose your own path). For more information, read the man page (also available online: http://manpages.ubuntu.com/manpages/hardy/man1/dolphin.1.html).

Help pages

The KDE applications have the help - with the Dolphin:

:~$ dolphin --help
Usage: dolphin [Qt-options] [KDE-options] [options] [Url] 

File Manager

Generic options:
  --help                    Show help about options
  --help-qt                 Show Qt specific options
  --help-kde                Show KDE specific options
  --help-all                Show all options
  --author                  Show author information
  -v, --version             Show version information
  --license                 Show license information
  --                        End of options

Options:
  --select                  The files and directories passed as arguments will be selected.
  --split                   Dolphin will get started with a split view.

Arguments:
  Url                       Document to open

Launch with the default application

The KDE has the "Default Appications" in the KDE system settings.

The kfmclient is:

:~$ kfmclient --help
Usage: kfmclient [Qt-options] [KDE-options] [KDE-tempfile-options] [options] command [URL(s)] 

KDE tool for opening URLs from the command line

Generic options:
  --help                    Show help about options
  --help-qt                 Show Qt specific options
  --help-kde                Show KDE specific options
  --help-kde-tempfile       Show KDE-tempfile specific options
  --help-all                Show all options
  --author                  Show author information
  -v, --version             Show version information
  --license                 Show license information
  --                        End of options

Options:
  --noninteractive          Non interactive use: no message boxes
  --commands                Show available commands

Arguments:
  command                   Command (see --commands)
  URL(s)                    Arguments for command

Opening the directory with the default file manager:

kfmclient exec /path/to/the/directory/

If you would like to open dolphin from your current path in your terminal just use,

dolphin . &