The official images install pkgin (which is a pkgsrc package, see
http://pkgsrc.se/pkgtools/pkgin) as part of the installation process. This requires manually downloading the pkgin package and its dependencies, then bundling them inside the installation media. Bundling pkgin by default won't be done when cross-compiling since it is a manual task. It's also not very documented right now.
Nevertheless, the lower-level pkg_add utility is available out of the box. Supposing you've set up networking, you can install pkgin with the command
PKG_PATH=http://minix3.org/pkgsrc/packages/3.4.0/i386/All/ pkg_add pkgin (from memory, I don't have a MINIX system at hand to test it right now).