On the current mainline (soon-to-be MINIX 3.3.0) X11 has not yet been updated, and thus is not yet available.
The following documentation as gone stale.
Introduction to X
X (also called X11 or, formally, the X Window System) is the standard windowing system for most UNIX systems. It is supported by MINIX 3, but not part of the basic distribution (due to its great size).
To use X, you must first fetch and install it. See InstallingBinaryPackages for more information on how to install packages with pkgin.
When installation is completed, be sure you are logged in as root and type:
You will get a login screen. Just log in with your MINIX 3 login name and password. The login screen will prevent you from logging-in if your password is empty.
After logging-in, the default window configuration will appear. You can modify the default using the .Xresources and .Xsession files, as described elsewhere in this handbook.
You can escape from the window by pressing CTRL+C. Immediately type "shutdown" to reboot and then start again after you added a user account. Otherwise the login window might keep trying to show itself even though you pressed CTRL+C.
Once the window manager password box is satisfied, it will start an X11 desktop. Note that this is not a full featured Linux install. There is no big list of programs or anything like that. Just the desktop and some shell terminals. If you need more programs you will need to install them or build them.
The following guides from The Linux Documentation Project give useful explanations of X and its components:
For MINIX releases 3.1.4 - 3.2.0, the official X documentation can be found below: